Merge search settings into Site Admin Configurations

[rhodges]

Remove 'search settings' table and instead include these fields in the site configuration form:

• [ ] Add new field(s) for search sensitivity to Configuriation.Configuration model
  • [ ] Values should be Choice fields with human-understandable options:
    • ('Strict', 1.0),
    • ('9', 0.9),
    • ...
    • ('Default', 0.6),
    • ...
    • ('2', 0.2),
    • ('Permissive', 0.1)
• [ ] Be sure to set the default values (from the default values in settings.py)
• [ ] Update code to use Configuration fields instead of 'search settings' model
• [ ] Create migration for creating the new fields on the model
• [ ] Create data migration to populate new fields from instance of 'site settings' (if exists)
• [ ] Create migration to drop 'site settings' model

[rhodges]

If 'data migration' piece is taking too long: skip it -- only two developers have records for this.