Changes include:

- Ensure Member.add_user is not called directly when not necessary
- New GroupMember.add_users_to_group to have the same abstraction level as for Project
- Refactor Member.add_user to take a source instead of an array of members
- Fix Rubocop offenses
- Always use Project#add_user instead of project.team.add_user
- Factorize users addition as members in Member.add_users_to_source
- Make access_level a keyword argument in GroupMember.add_users_to_group and ProjectMember.add_users_to_projects
- Destroy any requester before adding them as a member
- Improve the way we handle access requesters in Member.add_user
  Instead of removing the requester and creating a new member,
  we now simply accepts their access request. This way, they will
  receive a "access request granted" email.
- Fix error that was previously silently ignored
- Stop raising when access level is invalid in Member, let Rails validation do their work

Signed-off-by: Rémy Coutable <remy@rymai.me>

1. Change multiple updates to a single `update_all`

2. Use cascading deletes

3. Extract an average function for the database median.

4. Move database median to `lib/gitlab/database`

5. Use `delete_all` instead of `destroy_all`

6. Minor refactoring

- The normal seed creates all the data for cycle analytics the "right"
  way. It creates issues, merge requests, commits, branches,
  deployments, etc. This is good, but too slow for perf testing.
  Generating a 1000 sets of records this way takes more than an hour.

- When the `CYCLE_ANALYTICS_POPULATE_METRICS_DIRECTLY` environment
  variable is passed in, the seed only creates issues and merge
  requests. It then adds the `metrics` for each issue and
  merge request directly, to save time.

- The seed now takes about 4 minutes to run for 1000 sets of records.

1. These changes bring down page load time for 100 issues from more than
   a minute to about 1.5 seconds.

2. This entire commit is composed of these types of performance
   enhancements:

     - Cache relevant data in `IssueMetrics` wherever possible.
     - Cache relevant data in `MergeRequestMetrics` wherever possible.
     - Preload metrics

3. Given these improvements, we now only need to make 4 SQL calls:

    - Load all issues
    - Load all merge requests
    - Load all metrics for the issues
    - Load all metrics for the merge requests

4. A list of all the data points that are now being pre-calculated:

    a. The first time an issue is mentioned in a commit

      - In `GitPushService`, find all issues mentioned by the given commit
        using `ReferenceExtractor`. Set the `first_mentioned_in_commit_at`
        flag for each of them.

      - There seems to be a (pre-existing) bug here - files (and
        therefore commits) created using the Web CI don't have
        cross-references created, and issues are not closed even when
        the commit title is "Fixes #xx".

    b. The first time a merge request is deployed to production

      When a `Deployment` is created, find all merge requests that
      were merged in before the deployment, and set the
      `first_deployed_to_production_at` flag for each of them.

    c. The start / end time for a merge request pipeline

      Hook into the `Pipeline` state machine. When the `status` moves to
      `running`, find the merge requests whose tip commit matches the
      pipeline, and record the `latest_build_started_at` time for each
      of them. When the `status` moves to `success`, record the
      `latest_build_finished_at` time.

    d. The merge requests that close an issue

      - This was a big cause of the performance problems we were having
        with Cycle Analytics. We need to use `ReferenceExtractor` to make
        this calculation, which is slow when we have to run it on a large
        number of merge requests.

      - When a merge request is created, updated, or refreshed, find the
        issues it closes, and create an instance of
        `MergeRequestsClosingIssues`, which acts as a join model between
        merge requests and issues.

      - If a `MergeRequestsClosingIssues` instance links a merge request
        and an issue, that issue closes that merge request.

5. The `Queries` module was changed into a class, so we can cache the
   results of `issues` and `merge_requests_closing_issues` across
   various cycle analytics stages.

6. The code added in this commit is untested. Tests will be added in the
   next commit.

- The fixture generates data for every stage in the cycle analytics
  dashboard. Once this fixture has run, you shouldn't be seeing any
  "<not enough data>" messages for cycle analytics.

- This is probably not necessary for every fixture run, so it might be
  moved behind an env var in the future.