Add global search support

This is [global search branch](https://github.com/evernote/pootle/tree/feature/global-search) from Evernote fork, rebased on top of our master, with few fixes and indexed search enabled.

A staging server is available at http://feature-global-search.testing.locamotion.org/

I removed few unrelated commits, re-instated the indexed search and updated it to work with global search, however I have doubts about the need for indexed search; it seems to chock on large projects (searching from the home page on the staging server takes for ever) and the non-indexed search seems fast enough (try the exact search for example, since it is not indexed).