Moved Flask-WTF into lib/flask_wtf and using flask.ext.wtf for imports

No new versions or anything but for moving the existing ones to change the structure:

  * Moved the existing Flask-WTF into `main/lib/flask_wtf/` (instead of `main/lib/flaskext/wtf/`)

  * Using `flask.ext.foo` notation for importing:

    * For all the Flask-WTF imports

    * And also for the `login` and `auth` Flask extensions as used in `main/auth.py`; note that (for the time being) the actual implementations remained inside `main/lib/flaskext/` for these extensions