Update to support current versions of dependencies

Include support for Python 3, Flask-WTF >= 0.9.0, and Foundation 5.1.1.
Update version for pypi to 2.0 because of breaking changes. Specific
changes:

- Update to Foundation 5.1.1 in all relevant template files and static
  assets

- Change imports for Flask-WTF and WTForms per the guide at
  https://flask-wtf.readthedocs.org/en/latest/upgrade.html#version-0-9-0

- Replace Python 3 incompatible call to `iteritems` with `for key in
  dict` logic