Implements Flask-WhooshAlchemy search functionality

**Changelog**
- Adds __searchable__ to the models that require it
- Adds Flask-WhooshAlchemy==0.55 to the setup.py dependencies
- Adds user search form to the memberlist and the admin user management
- Adds whoosh_index calls on setup
- Adds UserSearchForm (users only) and SearchPageForm (users, posts, topics and forums)
- Adds a seperate Search page (available from the forum header)