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.

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

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

This column is not present in `db/schema.rb` and hence needs to be
removed conditionally.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/66901 for background