First iteration of the slash command menu UI on GitLab

Context

This issue is to bring the slash command menu UI to GitLab. For full context and iteration plan, see parent epic.

Proposal MVC

• Show slash command menu from gitlab-ui on GitLab. Probably only the following commands:
  • /reset
  • /clear
• Slash command menu shows only shows existing commands that would on every page, e.g. /reset
• Commands that do not accept arguments (e.g. free text) are sent as a question immediately upon selection from the menu. Slash commands do not fill the text input.

Related work that may impact this issue

Question/comment	Relevant link
There is work in flight to remove /clean, I assume that this command should not show on the menu	&14535 (closed)
FYI We want to change the existing /reset to /new_conversation. Nothing changes but the name of the command. Depending on what order the issues are picked up, it's possible that we show /new_conversation	#470818
FYI We want to add /help, depending on the order issues are picked up, it's possible that the menu should show /help	#462122 (closed)