Allow connection to Redis via unix socket, using
unix:/var/run/redis/redis.sock for example.

Default behaviour does not change, except that the full Redis URL must
be configured, with redis:// for tcp or unix: for unix socket.

changed installation overview list step #4
Gitolite -> GitLab shell 

Added creation of pids directory to fix #2995, replaced deprecated nginx init.d command with service one, some more clean-up

Made it a bit clearer what step 5 is for to stop people skipping it. This time with a relative link.

Change "th" to "the"

'th -> the' typo

Fixes #2334

Closes #2080
Fixes #2264