Add ability to search in issue description and issue comments

This is a proposition for improvement of search capabilities: in addition to merge requests and issue titles, search in issue descriptions and comments.

It is really frustrating to be limited to issue titles, hence the extension.

When searching in comments, the search is limited to notes attached to issues, and the results only show a link to the issue, not to the comment.

No particular management of duplicate links.

It is the quick and easy answer for suggestions http://feedback.gitlab.com/forums/176466-general/suggestions/3774012-improve-search-search-comments-in-issues-and-show and http://feedback.gitlab.com/forums/176466-general/suggestions/4315911-search-issues-description

What it does NOT provide (not limited to):
 * search by people
 * search in commits (as they are not in the database)
 * search by tag or id

Tests: I could not find the relevant tests for searches other than project names.