We request Gitaly in a N+1 manner to build discussion diffs. Once the diffs are from different revisions, it's hard to make a single request to the service in order to build the whole response.
With this change we solve this problem and simplify a lot fetching this piece of info.

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.

[10.6] Prevent notes on confidential issues from being sent to chat

See merge request gitlab/gitlabhq!2366
# Conflicts:
#	app/helpers/services_helper.rb

There are several quick actions now that don't need this access - /todo and
/unsubscribe for instance - but when these were first added, there
weren't. Quick actions are now responsible for checking their own permissions.

When the target is deleted, todos are destroyed, but we did not reset the todo
cache for users with todos on the deleted target. This would only update after
the next time the todo cache was updated for that user.

Signed-off-by: Rémy Coutable <remy@rymai.me>