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

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

This reverts commit 54a575f1, reversing
changes made to c63af942.

Move uploads to object storage

Closes #4163

See merge request gitlab-org/gitlab-ee!3867

Fixes #32282.

When a note is part of a discussion, the email sent out will be
`In-Reply-To` the previous note in that discussion. It also
`References` all the previous notes in that discussion, and the
original issue.

Closes gitlab-org/gitlab-ce#36054.

When a note is part of a discussion, the email sent out should be
`In-Reply-To` the previous note in that discussion.

Closes gitlab-org/gitlab-ce#36054

This allows EE to re-use the object, making it easier to trigger
indexing of the noteable.

This throttles the number of UPDATE queries that can be triggered by
calling "touch" on a Note, Issue, or MergeRequest. For Note objects we
also take care of updating the associated "noteable" relation in a
smarter way than Rails does by default.

Security fixes for 10.1 RC

See merge request gitlab/gitlabhq!2209

also fix some code styling issues

 - only show in merge-requests
 - show as a little glyph

this will remove the need make N queries (per-note) at the
cost of having to mark notes with an attribute

this opens up the possibility for other special roles for notes

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

This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/27070

Deprecate "chat commands" in favor of "slash commands"

We looked for things like:

 - `slash commmand`
 - `slash_command`
 - `slash-command`
 - `SlashCommand`

One should really use a separate table instead of using polymorphic
associations.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11168 for
more information.