Port CreateGithubPullRequestEvents migration from EE

See merge request gitlab-org/gitlab-ce!31802

Detect if pipeline runs for a GitHub pull request

When using a mirror for CI/CD only we register a pull_request
webhook. When a pull_request webhook is received, if the
source branch SHA matches the actual head of the branch in the
repository we create immediately a new pipeline for the
external pull request. Otherwise we store the
pull request info for when the push webhook is received.

When using "only/except: external_pull_requests" we can detect
if the pipeline has a open pull request on GitHub and create or
not the job based on that.

Since it is not possible to dynamically detect if a job is automatically
cancellable or not, a this new attribute is necessary. Moreover, it let
the maintainer of the repo to adjust the behaviour of the auto cancellation
feature to match exactly what he needs.

These are the structural changes for supporting the EE feature of moving
"code_owner_approval_required" state from existing on a project to being
on the protected branches individually, allowing for CODEOWNER
validation on push events.

Create index for users.unconfirmed_email

See merge request gitlab-org/gitlab-ce!32664

This speeds up the following query:

```sql
SELECT  users.* FROM users WHERE users.unconfirmed_email = ? ORDER BY
users.id ASC LIMIT 1
```

Presumably, this is a query coming from Devise.

Context is https://gitlab.com/gitlab-org/gitlab-ce/issues/66958.

This creates a partial index intended to speed up queries on
`ci_builds`. Particularly, `gitlab-monitor` has rather heavy queries.
Those have been changed to only look back 7 days and benefit from this
index tremendously.

Relates to
https://gitlab.com/gitlab-org/gitlab-exporter/merge_requests/101.

Creates new event when an epic is created, closed, reopened or
commented.

Remove Users.support_bot column

See merge request gitlab-org/gitlab-ce!32554