WIP: Improve slash commands placeholder UI

What does this MR do?

When comment content had slash commands, it tried to strip everything that came after / character which didn't go well when content had anything that involves / (eg; file paths, URLs and etc). This MR uses actual list of supported slash commands for the note and shows appropriate message related to slash command that was present in command. This bug affects the feature introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10760 which is part of %9.2.

Now user will see following messages while running slash commands.

• Single Slash Command: Applying command to <command description>.
• Multiple Slash Commands: Applying multiple commands.

Related EE port https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1865

Screenshots

Single Slash Command

Multiple Slash Commands

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • 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 & merge request numbers?

#32016 (closed)

https://gitlab.com/gitlab-org/gitlab-ce/issues/27614