Partially addresses #47424.

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.

Better group support in notes-related code

See merge request gitlab-org/gitlab-ce!18150

Updates notes-related services and rendering so this code can be
easily used for group-scoped resources (specifically Epics).

Related to gitlab-ee!5205

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

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

Add discussion API

See merge request gitlab-org/gitlab-ce!17510

* adds basic discussions API for issues and snippets
* reorganizes notes specs (so same tests can be used for all noteable types - issues, MRs, snippets)

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.