Adjust source fields: holding_institution, name, production_method, source_completeness

[dchiller]

NOTE: This PR removes some previously-committed migrations. You will need to re-create your local/staging database to do development after this PR is merged. I decided to do this because it will make the update of production go smoother (ie. not have to run to a certain migration, then run the migrate_records command, then apply the rest of the migrations), which ultimately felt preferable to prioritizing people's individual current development database status.

Makes significant changes to the Source model:

• removes the requirement for a Source to have a holding_institution. #1631 requests that holding_institution-less sources be possible only for reconstruction sources. This will be added once source_completeness field is available to mark reconstructions as such (see To-do's below).
• adds default [No Shelfmark] to Source.shelfmark field.
• adds the source_completeness choice field to replace the full_source boolean field and add the "Reconstruction" option. A command, populate_source_completeness, will populate the initial values of this field. Note that after this new field is populated, the full_source field will be redundant and can be removed (see To-do's below).
• adds the production_method choice field to note (currently) whether a source is a manuscript or printed source.

These changes are reflected in source create, edit, detail, and list pages. Relevant tests are also updated.

Makes cosmetic changes to the Institution model by changing various field defaults from latin abbreviations to english words.

Adds the source's name field to source create, edit, and detail views and to the source heading.

Performs some minor cosmetic changes: refactors forms.py a bit with a new StyledChoiceField class so that repeated changes to widget style attributes are no longer necessary and changes the ordering of the options in the "Holding Institution" input field on the Source admin page.

To-Do's

• run populate_source_completeness command on servers
• mark source reconstructions as reconstructions and fix orphaned sources (those sources that don't have proper institutions in the migration)
• add constraint to sources that require holding institutions unless they are reconstructions (completing #1631)

Closes #1628.

[dchiller]

The failing test will be fixed with the merge of #1643 -- once that is reviewed and merged I'll fix this.

[dchiller]

Tests are now passing. Have not heard back confirmation on some of the remaining formatting questions, so holding for the moment.