Partially addresses #47006.

A label name can contain a `_`, `~~`, or other Markdown-significant
characters. But label references are processed _after_ Markdown processing has
run, so we can't easily fix this on the backend.

We can make it more convenient, though, by changing the frontend to escape these
characters so they aren't processed as Markdown, when we insert them from
autocomplete.

Fixes #40620,#33678

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2878

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`

EE backport of
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1800

 - gl_emoji for falling back to image/css-sprite when the browser
   doesn't support an emoji
 - Markdown rendering (Banzai filter)
 - Autocomplete
 - Award emoji menu
    - Perceived perf
    - Immediate response because we now build client-side
 - Update `digests.json` generation in gemojione rake task to be more
   useful and  include `unicodeVersion`

MR: !9437

See issues

 - #26371
 - #27250
 - #22474

See !8975

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/26775