Support taxonomies instead of enumerations

[MikeThacker1]

Following discussion with Open Referral international people, whom we expect to accept the UK addition of a link_taxonomy table, we propose switching to using taxonomies rather than enumerations for:

• service.deliverable_type
• service.attending_type
• service. attending_access
• accessibility_for_disabilities.accessibility
• eligibility (already in OR UK in place of an enumeration)

This means that the only remaining enumerations will be:

• link_taxonomy.link_type (containing the above values as well as organization, eligibility, cost_option and service_area)
• service.status
• regular_schedule.freq