Explain label migration

See merge request !1025

Update changelog

Fixes #1509

See merge request !1026

cmake is required during the installation of the rugged gem

Fix spelling error in merge request performance issue warning.

Fix merge request API after #7526

Fix labels preview color when edit

This is related to #515. Parentheses are missing in coffeescript. For this reason, the color preview is not updated when loading the labels edit page.

See merge request !173

Label doc

Fixes #1487

See merge request !1028

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

API: Labels validation in issues API

Skipping rc3 because somebody (me) created a tag but did not bump the
version to rc3.

Fix project description on explore page, fixes #7458

use default white-space for sidebar buttons, fixes #7549

Fix color toggle for labels in edit view

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

Add oauth doc to 6.0 to 7.2 upgrader

Udate doc

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 dealing with newlines for editor to 5.1-to-6.0

Fix env variable style for documentation