Add command for indexing MEI files in solr

[dchiller]

This PR contributes to the re-implementation of MEI search in CU by adding the index_manuscript_mei command, which, given a manuscript ID, will parse and index the MEI for that manuscript in the solr index. The PR adds associated tests.

This PR also:

• add a submodule production-mei-files which tracks the production mei files repository which will be used to manage MEI files resulting from the OMR process
• changes location of test MEI files introduce in previous OMR-related commits (e.g. #845)
• removes the old import_mei_data command that was used for an older OMR-search process and is superseded by the new command
• makes some other development-related changes, including:
  • adds development dependencies (stubs, etc.) to the project dependencies
  • explicitly exports Django models to conform the linting standards

[dchiller]

Great suggestions...I've push some changes that add exceptions at these places.