Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33594

Was supposedly fixed in MR for
https://gitlab.com/gitlab-org/gitlab-ce/issues/32086 but seeing this pop
up again

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`

#22827

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

- No longer depends on the "hidden checkbox".
- No longer depends on manually storing/restoring the scroll position.
  Instead, we take advantage of jquery.scrollTo.
- Event-based.
- Simplifies the state-based styling.

In these particular instances we can just use HAML tags directly. This
can shave off some time spent loading issue pages, though this depends
on the amount of comments being displayed.

When viewing https://gitlab.com/gitlab-org/gitlab-ce/issues/2164 locally
these changes reduce loading time by about 400 ms in total.

This reverts commit c39c8234.