Consolidate functionality shared between Issue and MergeRequest

ability to preview notes

Fix resque worker becoming paused on restarts

Also adds specs to the two affected classes that would have caught my
dumb mistake.

Any associations, validations, delegates, scopes and methods that
were exactly the same in both Issue and MergeRequest models have been
moved to a new IssueCommonality module (role) that gets included by each
class.

There was actually quite a bit of duplication, because MergeRequests are
basically just specialized Issues.

Gitlab flavored markdown

Add automatic HTML id attributes to markdown headings to allow linking to content sections in wiki

Hook url should be able to contain http basic authentication

Fix UMASK checking in .gitoliterc

Grammar fix after creating a project