Rename "Slash commands" to "Quick actions"

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2012

• Rename "Slash commands" to "Quick actions"
  • Maintain Mattermost slash commands
  • Maintain Slack slash commands
• Deprecate "chat commands" in favor of "slash commands"

Touches docs, comments, JavaScript and Ruby

Are there points in the code the reviewer needs to double check?

slash_commands and chat_commands existed in the codebase before. I renamed slash_commands -> quick_actions and chat_commands -> slash_commands given instructions "deprecate chat commands name" in the maintain slash commands section but perhaps I misinterpreted. See https://gitlab.com/gitlab-org/gitlab-ce/issues/27070 - Perhaps we want to consolidate under the same name.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27070 (closed)