Bump required gitlab-shell version to 1.9.7

See merge request !1031

Add installing cmake to 7.2 update docs

See merge request !1029

Add dependency info for mysql-postgresql converter

upgrade ruby and gitlab shell config during 6.0->7.1 & 6.0->7.2 upgrades

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

Running converter with Python 2.6.x installed gives the following error:
AttributeError: 'module' object has no attribute 'check_output'

After installing Python 2.7.x everything works fine.

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

ruby 2.0.x required for gitlab 7.0+

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