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.

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

This adjusts the partial condition for an index. The index is intended
to be used when counting active users with `ghost IS NOT TRUE AND
bot_type IS NULL`.

With the current index, this wasn't working as the partial condition
didn't match the query: `ghost <> TRUE` is not semantically equivalent
to `ghost IS NOT TRUE` (null semantics).

The reason we add an index particularly intended for EE is that the EE
query is going to have the additional part `AND bot_type IS NULL`
whereas the CE query doesn't. Logically, it'd be enough to have an index
for `ghost IS NOT TRUE`. However, on GitLab.com, the query planner makes
poor choices when the additional `AND bot_type IS NULL` part is present:
It goes for the index on `bot_type` and doesn't use the partial index.

Note the existing index isn't being used at all according to GitLab.com
index statistics. Hence we can first remove it and don't have to worry
about the window of time without an index.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/66770

Rename epic column state to state_id to be consistent with
issues and merge requests