Add `/assign me` alias support for assigning issuables to oneself

What does this MR do?

Currently, when a user wants to assign an issue/MR to himself, he needs to type his full username or select it from the suggested ones in the dropdown list.

This commits suggest a faster solution which is typing /assign me

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

I hesitated about where to introduce this change. I submitted the simplest solution supporting /assign me or /unassign me, but we could also support more complex commands like assign me @jdoe, or even @me as a default "alias" to the current user in user references parser.

I'm not yet that familiar with GitLab code base to have a better opinion on this.

Why was this MR needed?

It's a suggested enhancement for /assign command

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• 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?

#35304 (closed)