add a 'warning' level for custom validations
Split all mtime/ctime fields into a separate migration helper
Switch last_modified to system_mtime

Add a new 'user_mtime' to track modifications that were made by a user directly
modifying a record (as opposed to the system bumping system_mtime to encourage
reindexing)
Frontend changes
Merge pull request #455 from hudmol/system-mtimes

System mtimes
Remove forced placeholders for date widgets (can be set now in locales '_placeholder'); Also remove the auto calendar widget and now offer a button to display the widget on demand;
WIP backend support for batch creation of locations
Add a dry_run to location batch creation
WIP frontend changes for location batch form
When checking for empty elements, treat empty hashes as empty too.
Implement batch creation of location in the frontend (and tweak the backend to play nice)
Add some warning messages and a spinner to the location batch form
Add selenium tests for the new batch import UI
Merge pull request #454 from hudmol/47182143-make-otherlevel-less-required

add a 'warning' level for custom validations
Merge pull request #456 from hudmol/47583529-remove-auto-date-widgets

[Deliver #47583529] remove auto calendar widget and placeholders from date fields
Merge pull request #457 from hudmol/batch-locations

[Delivers #43561173] Batch creation locations
Remove unused agent controller
Define permissions for all endpoints (previously marked as .nopermissionsyet)

Get around the thorny issue of who should be able to update/delete subjects,
agents and vocabularies with the following rules:

  * You can update agents/subjects/vocabularies if you're granted access to
    update archival records in any repository.

  * You can delete agents/subjects/vocabularies if you're granted access to
    delete archival records in any repository.
Refactor search to avoid the need for a single global endpoint.

Instead use type-specific endpoints with more fine-grained permissions.
Remove superflous determinant_permissions array