[Documentation] GitLab 7.2 requires GitLab shell 1.9.7

Fix installation version, we have 7.2 now

Change the branch name of cloning source to 7.2!! This also needs to be cherry picked to 7.2 stable branch

/cc @JobV @jacobvosmaer @marin

ps: unfortunately, there is mo 🤘 gemoji :(

See merge request !174

Protect unsubscribe path

See merge request !1033

Backticks cause quote

See merge request !704

Swap is required on servers with 1GB RAM

See merge request !1032

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