Updated to Flask-WTF 0.9.3

Updated to Flask-WTF 0.9.3; note that this requires us to import `wtforms` and `wtforms.validators` ourselves: see https://flask-wtf.readthedocs.org/en/latest/upgrade.html#version-0-9-0 reading: *"Dropping the imports of wtforms is a big change, it may be lots of pain for you, but the imports are hard to maintain. Instead of importing Fields from Flask-WTF, you need to import them from the original wtforms"* (and this applies to the validators as well).