Upgrade flask-admin to v2

[russss]

There's an alpha version at the moment which should be usable so I'm inclined to go for it at this point.

The existing version is quite old and is generating a bunch of deprecation warnings.

I took a look at upgrading and there are a few breaking changes which are not especially obvious to fix. I don't think it's too much work, but it's non-trivial.

[ElLorans]

The only breaking changes I can think of are:

1. how to initialize the app. In 1.6.1 you would do admin = Admin(app, name='microblog', template_mode='bootstrap3') while now it's admin = Admin(app, name='microblog', theme=Bootstrap4Theme())
2. Standardised app.config names (they now all start with FLASK_ADMIN: Use FLASK_ADMIN_ prefix for all app.config configuration pallets-eco/flask-admin#2472 .

The dev branch on github has type hint support which should further make the transition easier. If you find any difficulties, please raise an issue and we'll include in the docs. It's been 1 year of work on v2, but we are almost there...

[wlcx]

The wtforms upgrade has borked our flask-admin pages due to pallets-eco/flask-admin#2318, another good reason to upgrade

[ElLorans]

You can use pip install flask-admin==2.0.0a4. The alpha version is stable and production ready, we are finalizing to publish 2.0 this year.