closes #8718, closes #8721

Similar to https://github.com/gitlabhq/gitlabhq/blob/master/doc/install/database_mysql.md clarify that `template1=#` is part of the prompt.

Add documentation changes to:

 * Set the permissions of the unix socket properly
 * Create the directory for the socket
 * Make sure tmpfiles.d persists said directory

The first change makes sure gitlabl can connect to the socket. The latter two
avoid redis from failing to start on systemd-based systems, including recent
versions of Debian.

Fixes
https://github.com/gitlabhq/gitlabhq/commit/2b35c887904b6a7a05a7c88fdf1ccd66ba943e20#commitcomment-7829537,
https://github.com/gitlabhq/gitlabhq/commit/5704e6fee9d2dea67c4a91b43d06a9958484d29b#commitcomment-7829367,
&
https://github.com/gitlabhq/gitlabhq/commit/5704e6fee9d2dea67c4a91b43d06a9958484d29b#commitcomment-7829381

This patch changes default permission of the gitlab/log directory to
u+rwX,go-w. This is done to make the directory NOT readable by group and
others and to avoid logrotate complaining about it. chmod 755 is not
used to avoid setting executable bit on file within the log dir.

fix formatting issue with self-signed certificate section

also adds comment about updating nginx files during upgrades

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

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