Make all internal plugins inherit from plugin base classes (and optionally extract)

[sdruskat]

Currently, some internal plugins don't inherit from the plugin base classes, e.g., the Invenio postprocess ones.

In the interest of dogfooding and consistency, all plugins should inherit from the respective base classes IMHO.

Optionally, these plugins could also be extracted into separate repos, although I'm not fully convinced that this is necessary.