Refactor web editor

* fix problem with editing non-master branch
* before commit make sure branch exists
* dont allow user change file in one branch and commit to another existing branch
* remove a lot of code duplication
* remove outdated statellite errors
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixes #1761

See merge request !773