Added authentication service for docker registry

This adds a simple authentication service for docker which uses current user credentials to authenticate pulls and pushes.

I have only one concern. Since the `.docker/config` is unencrypted, thus the password for user stored there is unencrypted, maybe we should from the start implement function to generate/provide a separate password just for the purposes of accessing docker registry?

What do you think @jacobvosmaer @sytses @marin?

cc @marin 

See merge request !3787

Add cache count metrics to rails cache

See merge request !4157

Change colors for charcoal and graphite sidebar themes
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

cc @skyruler @jschatz1 

See merge request !4149

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Refactor diff notes

Related to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4101

/cc @rspeicher 

See merge request !4110

# Conflicts:
#	config/initializers/1_settings.rb