This commit changes the colours of languages in the language graph to
the ones Linguist has defined. When there is no colour defined for a
language in Linguist, it will fall back to the old method of finding
a colour.
Build related UI fixes
## Before

## After

See merge request !2015
It is now possible to view contributor and commit statistics for
refs beyond the default branch.
Additionally, conditions in RefsController#switch have been refactored
in order that a `case` block is used now.
* remove commits stats page (will be replaced by graphs)
* remove additions/deletions from graph because of bad performance
* keep graph logic in gitlab
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>