Bump WTForms and Flask-WTF to latest versions

WTForms versions less than 3.0.0 have a security vulnerability where
arbitrary HTML can be inserted into the label of a form, allowing the
possibility of a cross-site scripting attack.

I don’t know if there’s anywhere we put user-generated content into form
labels but it’s possible we are vulnerable somewhere.

This require moving some imports because as of
https://github.com/wtforms/wtforms/pull/614/files
there is no longer a separate module for HTML 5 fields, they are now
considered core fields.

As of https://github.com/wtforms/wtforms/issues/445/files custom
implementations of `pre_validate` or `post_validate` must raise
`ValidationError` to trigger a validation message, where we were raising
`ValueError` this was no longer being caught.

As of https://github.com/wtforms/wtforms/pull/355/files `StringField`
returns `None` for empty data, not `''` but our `validate_email_address`
function only accepts strings.

app/main/validators.py

@@ -58,7 +58,7 @@ class ValidEmail:
 
     def __call__(self, form, field):
 
-        if field.data == '':
+        if not field.data:
             return
 
         try: