Flask Mongoengine WTForms update

[noirbizarre]

This PR supersedes #586, #654, #682 and #767 because they were dependent upgrades.
These PR have been merged into this one and all required fixes/refactorings applied:

• flask_wtf.Form is now flask_wtf.FlaskForm and has a little signature change
• flask_mongoengine has a new connection manager
• wtform-json use StringField instead of the deprecated TextField which breaks every StringField instance (inherited or not) using default value (pending bug fix: Fixes (Python 3.6, tox, StringField default, latest versions...) kvesteri/wtforms-json#49), casting value to something else than a string or expected None as default

Consequence:

• make use of mongoengine.get_db instead of the removed flask_mongoengine.connection.get_db
• cleanup the form fields dependency tree: all fields expecting None as default/empty, inheriting from StringField but returning another type or using a default value now use udata.forms.field.Field as base class
• Form(formdata, obj) calls are now Form(formdata, obj=obj)

[davidbgk]

👍

[davidbgk]

#noqa still valid?

[noirbizarre]

I was happy too to remove it but sadly this is here because of the mandatory early wtforms_json.init()

[davidbgk]

Remaining print statement.