Fix color toggle for labels in edit view
cmake is required during the installation of the rugged gem

The following error is encountered during the installation of the rugged gem:

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.1 extconf.rb
    checking for cmake... no
    ERROR: CMake is required to build Rugged.
    *** extconf.rb failed ***
Add 7.1-to-7.2.md update guide
Add 6.0-to-7.2.md guide ahead of 7.2 release
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
Test if the label is valid before creating link
Remove invalid characters from the label name
Use the default color when migrating labels
Merge branch 'fix_label_migration' into 'master'

Fix label migration

Failure to create any labels at all caused this DB migration to run for over 40
minutes during a gitlab.com deploy this morning. This change specifies a color f
or each new label and strips invalid characters from the label name.

See merge request !1024
Bump version to 7.2.0.rc2
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
label documentation
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.