[FEATURE] Extract Fedora stuff to its own module

[rosiel]

Overview of feature request

We claim that Fedora is optional, but the islandora module assumes, in many places, that you have Fedora.

Components to move to the new Fedora module (and perhaps corresponding recipe?):

• flysystem Fedora integration code (src/Flysystem as well as islandora.services.yml)
  • configurations that set media files to go to Fedora can be in a Recipe
• the "Fedora URI"/"gemini_pseudo_bundles" code (islandoraSettingsForm.php, islandora.module, islandora.schema.yml, islandora.settings.yml)
  • This should also take with it the dependency on islandora/fedora-entity-mapper
• the system.action.index_ENTITY_in_fedora (and blazegraph) and system.action.delete_ENTITY... and system.action.index_file_as_fedora_external_content should be removed to a recipe.
• the view of non-fedora files could also go in a recipe.

There's a snag in that our EventGenerator, which is part of code that is more about getting derivatives and indexing with Alpaca (already extracted as Huacaya) contains a small bit of code that references Fedora. However it seems innocuous if you don't have flysystem or fedora installed(?).