Attachment URL with non-/ relative root

The attachment URL was not working with relative_url_root not equal to '/'. I suggest this fix.
append .xmlschema to system hook timestamp
Remove unused title parameter.

It is always overridden by to humanized source branch name by
MergeRequests::BuilService, which is used by new_project_merge_request_path.
Handle undefined text area values

Check to see if a text area's `val` is defined before trying to call
`replace()` on it.
add sign-in count

add sign-in count to admin section profile view
Add article about GitLab flow.
Add GitLab Flow to the index.
Fix grammar mistakes, thanks Angus
Fix spelling mistakes, thanks Nico Hamm.
Fix spelling mistakes, thanks Achilleas Pipinellis and Stephan van Leeuwen.
Fixed items mentioned by Achilleas Pipinellis, thanks.
Rewrite this section after suggestion from Achilleas.
add HSTS Policy warning

Add warning about HSTS header as it means user will need to provide secure connection access to site for next 24 months from page view. See https://en.wikipedia.org/wiki/HTTP_Strict_Transport_Security for more details.
Adding image descriptions and links as well as consolidating the link and close issues text.
Four stages.
It's the shell.
Fix problems identified by Achilleas Pipinellis, thanks Achilleas.
Insert at a random point in the changelog to prevent conflicts.
Source repo is now clickable.
Change gitlab/log permissions in installation.md

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.