Closes #45412

This is called repeatedly when viewing a merge request, and this should
improve performance significantly by avoiding shelling out to git every time.

This should help https://gitlab.com/gitlab-com/infrastructure/issues/4027.

Closes #45247.

Closes #45326

Revert "Merge branch '44870-remove-extra-space-around-comment-form-on-merge-requests' into 'master'"

This reverts merge request !18120

Since !18150 abstract reference filter supports also `group`
as a resource parent (not only `project`). Some reference filters,
which inherit from abstract filter, depend on parent being `project`
when checking a reference.

This patch adds explicit check of parent class to the given filters.

Fixes gitaly#1126

This refactors the Markdown pipeline so it supports the rendering of
multiple documents that may belong to different projects. An example of
where this happens is when displaying the event feed of a group. In this
case we retrieve events for all projects in the group. Previously we
would group events per project and render these chunks separately, but
this would result in many SQL queries being executed. By extending the
Markdown pipeline to support this out of the box we can drastically
reduce the number of SQL queries.

To achieve this we introduce a new object to the pipeline:
Banzai::RenderContext. This object simply wraps two other objects: an
optional Project instance, and an optional User instance. On its own
this wouldn't be very helpful, but a RenderContext can also be used to
associate HTML documents with specific Project instances. This work is
done in Banzai::ObjectRenderer and allows us to reuse as many queries
(and results) as possible.

By default this was an OPT_IN RPC, and now graduates to OPT_OUT as the
acceptance testing was successful in:
gitaly#1009

This follows the conventions of our [migration
process](https://gitlab.com/gitlab-org/gitaly/blob/master/doc/MIGRATION_PROCESS.md)

By default this was an OPT_IN RPC, and now graduates to OPT_OUT as the
acceptance testing was successful in:
https://gitlab.com/gitlab-org/gitaly/issues/1008

This follows the conventions of our [migration
process](https://gitlab.com/gitlab-org/gitaly/blob/master/doc/MIGRATION_PROCESS.md)

* Use the correct key prefix
* Clear old cache keys

TODO:

At some point we could remove clearing old cache keys.

This reverts merge request !17730

[ci skip]

This removes the extra check for project-ids which is not needed at all.
This does not necessarily reduce execution time of the query, but
improves planning time by a few millseconds.

Closes #37125.