Support custom themes better

[samuelhwilliams]

I tried to implement a custom theme for Flask-Admin (https://github.com/samuelhwilliams/xgovuk-flask-admin/) and ended up needing to do data manipulation in quite a few places; we should make it easier for themes to worry only about visual/styling issues rather than flask-admin data internals.

[princerb]

#2647 (comment)

[hasansezertasan]

Hey @samuelhwilliams, you've got some ideas about custom theme adoption. WDYT about #2847, would it make this task easier?