Custom mentions implementation

[rafalp]

This PR adds custom mentions implementation in place of jQuery.atWho.

Fixes #1484

TODO

• Open popup for mention at beginning of post
• Open popup for first mention in first line
• Open popup for second mention in first line
• Open popup for first mention in nth line
• Open popup for second mention in nth line
• Close popup on click/touch outside of mentions container
• Close popup on editor scrooling
• Respect editor's vertical and horizontal scrolls in top/left calculation
• Display popup above mention if user to mention is at bottom of editor area
• Check how superlong words are treated by editor
• Populate popup with suggestions
• Fix textarea not being 100% height due to being in container
• Remove atWho library

Keyboard controls

• If popup is open, up/down arrows should change active index
• If popup is open, ender should replace mention under caret with new one
• If popup is open, esc should close it

[rafalp]

This is not a blocker for 0.30 release. If I fail to finish it, I will release 0.30 without custom mentions and delay this change for next feature release.

[rafalp]

Proof of concept work:

misago-mentions.mp4