Fix color toggle for labels in edit view
Update omniauth-google-oauth2 gem

Udate doc
Mention Google OAuth API change in update guides

Add oauth doc to 6.0 to 7.2 upgrader
use default white-space for sidebar buttons, fixes #7549
Restyle explore projects to fix #7548
Improve labels validation and expose error messages
Add spec on labels validation
Merge pull request #7536 from yuters/patch-5

Fix color toggle for labels in edit view
Merge pull request #7552 from Razer6/fix_star_button

use default white-space for sidebar buttons, fixes #7549
Merge pull request #7553 from Razer6/fix_project_description

Fix project description on explore page, fixes #7458
Bump version to 7.2.0.rc4

Skipping rc3 because somebody (me) created a tag but did not bump the
version to rc3.
Merge pull request #7526 from jubianchi/api/issue-api-validate-labels

API: Labels validation in issues API
Merge branch 'fix_google_oauth_gem' into 'master'

Update omniauth-google-oauth2 gem

#### What does this MR do?

This PR updates google omniauth gem.

#### Why was this MR needed?

This is needed, because google deprecates some API on Sept, 1st.

All tests pass, but I still have to ~~test and update documentation in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/integration/google.md~~

**Contacts API" and "Google+ API need to be enabled**

This need to be stated in the update guide (7.1 to 7.2), as well as (6.0 to 7.2)

This fixes #512

See merge request !171
Bump version to 7.2.0.rc5