UI improvements mostly for mobile screens

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Smaller tabs for mobile view

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'improve-mobile-ui' into 'master'

Improve mobile UI pt2

Next UI elements were improved for mobile view

* comments
* comment form
* issues navigation
* removed duplicate MR sidebar btn
* tabs

Desktop improvements:

* increase comment author username font size
* dark colour for comment author name

Related to #1493

See merge request !1285