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.

Adjust routable metric

See merge request gitlab-org/gitlab-ce!32591

* Define the counter once.
* Make metric name follow Prometheus naming conventions.
* Add label for call method.

- Add new service that transfers milestones
from a group to a project
- Include new service in Projects transfer service
- Include FromUnion module in Milestone model
to use in transfer service
- Add specs for new milestones service
- Add specs for transferring milestones in
project transfer service

Improve diff lines count efficiency

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

This fixes a high frequency N+1 issue:

`RoutableActions#find_routable!` is used across many controllers to
retrieve e.g. the Project or Namespace by path. The `#find_routable!`
method calls `#ensure_canonical_path` which in turn retrieves
`#full_path` from the given Routable.

This in turn triggers a lookup on `routes`, leading to a high frequency
of these queries:

```sql
SELECT  "routes".* FROM "routes" WHERE "routes"."source_id" = $1 AND
"routes"."source_type" = $2 LIMIT $3
```

This is unnecessary as we already join `routes` in
`Routable#find_by_full_path` anyways.

Modified schema via migrations.
Added one-to-one relationship between the two models.
Added changelog file

Expose `name` and `merge_request_event_type` in serialized json and predefined variables

See merge request gitlab-org/gitlab-ce!32323

Add system hooks for project/group membership updates

Closes gitlab-ee#12252

See merge request gitlab-org/gitlab-ce!32371