Make (Form|View)ModeAccessCheck use the entity display access control handler

Problem/Motivation

Once #2866666: Add proper access handlers for view and form displays → is in, the code in (Form|View)ModeAccessCheck could leverage the entity display access control handler.
There is quite some moment in (Form|View)ModeAccessCheck though, which has to be untangled.

Proposed resolution

Try to untangle it.

Remaining tasks

User interface changes

API changes

Data model changes