feat: add neume exemplars for omr neume search

[dchiller]

This PR reintroduces neume exemplars for search. Closes #878. With this, we can also close #796.

This PR:

• removes the pick_neume_exemplar command and replaces that process with a visual interface for picking neume exemplars. A screenshot of the interface is below.
• adding an order attribute to neume exemplars so we can have them appear consistently in a consistent order. Also refactors the admin_image property for the neume exemplar (this property defines the url for an image that appears in the admin panel for a neume exemplar -- screenshot below).
• adds serializers and views for:
  • getting examples of neumes in a source (NeumeSetAPIView)
  • an API endpoint for getting or setting existing neume exemplars in a source (NeumeExemplarsAPIView)
  • a template view to power the interface for picking neume exemplars (PickNeumeExemplarsView)
• customizes the admin templates for the Django Manuscript model admin to include a link to the interface for picking neume exemplars (see screenshot below)
• makes adjustments to the frontend for displaying these neume exemplars in the search process (see screenshot below)
• adds relevant tests for the above

Additionally, removes an unused closing span tag in the base template.

Neume exemplar admin image

Interface to pick neume exemplars

Link to pick neume exemplars on Manuscript admin page

Neume exemplars in the search interface