Requalified users.is_admin flag as legacy accessor to admin actions in case token lacks admin scope(s).

Related to #85

Everything should now be configured at runtime, with the exception of
the `Pleroma.HTML` scrubbers (the scrubbers used can be
changed at runtime, but their configuration is compile-time) because
it's building a module with a macro.

Same API as the old Pleroma.Config

Discussed in !426 (comment 7232)