Improve MetadataForm module registration

[petrkalos]

Current implementation of MetadatForms module is breaking data.all's code architecture by making an optional module (metadata_forms) being a dependency of core modules which shouldn't be possible but is being enforced by introducing a call proxy (metadata_manager). Instead the problem can be solved in the two following ways

1. MF are enabled per module with a feature flag. Then initializer of each module is responsible for conditionally registering to MF. This approach would still spread MF.register() across the project
2. MF are still enabled globally but during initialization of the module we iterate over all the modules/classes/models we want to enforce MF. The caveat is that MF module should be loaded after all the modules we want to enforce MFs. To avoid enabling a disabled module we must first check if it's enabled and then define it as a dependency.

Originally posted by @petrkalos in #1730 (comment)