feat: mentioning users & issues/pulls #77

When typing '@' you should be able to mention users, and also notify them. '#' should reference issues/pulls.

A auto complete should appear when typing those. For users, auto complete should show repo collaborators and everyone who commented to the active issue/pull thread and people you follow, in addition to the exact user typed.

For issues/pulls, the auto complete should only show ones from the current repo. Issue/pull mention should show the title of the issue/pull alongside an icon indicating if it's opened, closed or merged (this should also work for foreign repositories issues/pulls, but should also show the repo owners and the repo name).