[Documentation] more cleanup installation guide formatting

[Documentation] add nginx test

* fix code blocks
* Make capitalization of titles consistent - missed some in
https://github.com/gitlabhq/gitlabhq/pull/7576

Set charset encoding to UTF-8 for snippets

change X-Frame-Options to SAMEORIGIN for nginx SSL config

Add 'unsubscribes' to the paths blacklist

This is to prevent issues with the admin email feature in GitLab EE.

See merge request !1034

Always set satellite origin

See merge request !1036

Import the database as the `git` user

See merge request !762

Cleaner UI for login and signup pages

![Screenshot 2014-08-15 17.51.52](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/fc8bf3c4a4/Screenshot_2014-08-15_17.51.52.png)
![Screenshot 2014-08-15 17.52.39](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/72706f55b0/Screenshot_2014-08-15_17.52.39.png)
![Screenshot 2014-08-15 17.51.33](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/c46f972944/Screenshot_2014-08-15_17.51.33.png)

See merge request !1022

Unify Nginx files to match as closely as possible

clarify which configs should be updated

remove prerelease note from 6.0 to 7.2 guide

needed to allow sidekiq to load on background jobs tab

[Documentation] cleanup installation guide formatting

make configuration comment spacing consistent

[Documentation] GitLab 7.2 requires GitLab shell 1.9.7

* Fix some formatting issues
* Make capitalization of titles consistent
* Update image that shows how to select branch

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

This prevents issues with satellites containing outdated origin remotes
after administrators move the git repositories directory.

GitLab EE has a /unsubscribes/ route.

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