/reassign should only work when multiple assignees are available. And
since this is only the case for GitLab EE, remove it from CE.
This was picked using 517c97eb12e44f354787313dd35ac7a1642f5b20 of gitlab-ee.
The /reassign quick command works even when no multiple assignees are allowed of
there isn't any assignee yet. So for consistency, it's also be backported to
CE. But it functions the same as the /assign quick action.
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`
