Add autocomplete for quick action /status

When you type /status you should be presented with a list of available statuses in a dropdown. On selection the selected status name should be added to the text.

1. GFM auto complete logic is available in app/assets/javascripts/gfm_auto_complete.js
2. Added to markdown field via gl.GfmAutoComplete?.dataSources, for example app/assets/javascripts/notes/components/comment_form.vue:93
3. FE calls internal endpoint that resolves entities. For example see app/controllers/projects/autocomplete_sources_controller.rb. This also exists for groups and for ee pendants.