Other improvements:
- Ensure slash commands autocomplete doesn't break when noteable_type is not given
- Slash commands: improve autocomplete behavior and /due command
- We don't display slash commands for note edit forms.
- Add tests for reply by email with slash commands
- Be sure to execute slash commands after the note creation in Notes::CreateService

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

- Return only slash commands that make sense for the current noteable
- Allow slash commands decription to be dynamic

Other improvements:

- Add permission checks in slash commands definition
- Use IssuesFinder and MergeRequestsFinder
- Use next if instead of a unless block, and use splat operator instead of flatten

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

Some important things to note:

- commands are removed from noteable.description / note.note
- commands are translated to params so that they are treated as normal
  params in noteable Creation services
- the logic is not in the models but in the Creation services, which is
  the right place for advanced logic that has nothing to do with what
  models should be responsible of!
- UI/JS needs to be updated to handle notes which consist of commands
  only
- the `/merge` command is not handled yet

Other improvements:

- Don't process commands in commit notes and display a flash is note is only commands
- Add autocomplete for slash commands
- Add description and params to slash command DSL methods
- Ensure replying by email with a commands-only note works
- Use :subscription_event instead of calling noteable.subscribe
- Support :todo_event in IssuableBaseService

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

So we extend Gitlab::Email::Receiver for this new behaviour,
however we might want to split it into another class for better
testing it.

Another issue is that, currently it's using this to parse project
identifier:

    Gitlab::IncomingEmail.key_from_address

Which is using:

    Gitlab.config.incoming_email.address

for the receiver name. This is probably `reply` because it's used
for replying to a specific issue. We might want to introduce another
config for this, or just use `reply` instead of `incoming`.

I'll prefer to introduce a new config for this, or just change
`reply` to `incoming` because it would make sense for replying to
there, too.

The email template used in tests were copied and modified from:
`emails/valid_reply.eml` which I hope is ok.

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil