Add Tab autocomplete for username mentions across the platform

Summary

It is very nice that we have autocomplete for users and such. But it is sad that we still require a mousse to click on them. It should be posible with using tab, just like in an IDE to automatically insert the user you want to mention. This must be conistently done trough the whole platform. It is better for usability and comfort.

Steps to Reproduce

Not provided.

Expected Behaviour

Pressing the Tab key while a username autocomplete suggestion is visible should automatically insert the selected user mention, similar to IDE behavior.

Actual Behaviour

Currently, users must click with a mouse to select a username from the autocomplete dropdown.

Environment

Not provided.

Reported by retoor via DevPlace.