When searching in issues and merge requests include the comments

Reported by Sven Strickroth of TortoiseGit fame

Right now only the title and description is searched, we should search the comments too.

For example: When I search for 'git flow' https://gitlab.com/tortoisegit/tortoisegit/issues?utf8=%E2%9C%93&issue_search=git+flow&state=all&scope=all&assignee_id=&author_id=&milestone_id=&label_id= I expect to find issue tortoisegit/tortoisegit#992 with 'git flow' in the results

Maybe we can use part of http://blog.lostpropertyhq.com/postgres-full-text-search-is-good-enough/

What do you think @dzaporozhets ?

/cc @JobV @rspeicher