Fix could not mention members when editing merge request

What does this MR do?

Fix could not mention members when editing merge request.

There is not problem when new a merge request or issue.

Issuable form using a var called gl.GfmAutoComplete.dataSources to request backend resp, then show to users.

gl.GfmAutoComplete.dataSources which is defined in _init_auto_complete.html.haml by calling a helper autocomplete_data_sources.

autocomplete_data_sources requires two arguments: object, noteable_type., noteable_type => noteable.class needs to be defined in controller.

Relevant files

app/assets/javascripts/issuable_form.js
app/helpers/application_helper.rb
app/views/layouts/_init_auto_complete.html.haml

Relevant commit: https://gitlab.com/gitlab-org/gitlab-ce/commit/5c2078838bb9de710f9025513c4b6ec664bba313

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

Why was this MR needed?

User could not mention other members when they editing merge request.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?