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).

In addition, the `Form` needs to be imported from `flask_wtf` (instead of `flask.ext.wtf`).

This is documented in the CHANGESET file, for those upgrading from older versions to 0.9.0