feat!: remove blockstore support + 0.3.0 version bump (#24)

Blockstore was completely removed from edx-platform in:
  openedx/edx-platform#34739

As a follow-on to that work, this commit removes the entirety of two
blockstore-related plugins (tutor-contrib-blockstore-filesystem and
tutor-contrib-blockstore-minio), as well most blockstore references
present in tutor-contrib-library-authoring-mfe.

The BLOCKSTORE_COLLECTION_UUID configuration value was left in place
because it's expected by the Library Authoring MFE and currently
required by the Django view that creates Libraries (even though the
value is never used). We will remove those references in a later PR.

README.rst

@@ -20,8 +20,6 @@ Plugin                               Status (*Experimental*, *Production*, or *D
 ===================================  ======================================================
 tutor-contrib-learner-dashboard-mfe  Deprecated
 tutor-contrib-library-authoring-mfe  Experimental
-tutor-contrib-blockstore-filesystem  Experimental
-tutor-contrib-blockstore-minio       Experimental
 ===================================  ======================================================
 
 Getting Started
@@ -31,7 +29,7 @@ Generally, assuming `Tutor is installed`_, to add a plugin in this repository
 to your deployment environment you would:
 
 .. code:: bash
-	  
+
 	  git clone [email protected]:openedx/openedx-tutor-plugins
           cd openedx-tutor-plugins/plugins/<plugin-name>
           pip install -e .