Updated Flask-WTF to 0.9.3

Updated Flask-WTF to 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).

This is documented in the CHANGESET file, for those upgrading from older versions to 0.9.0