Create mappings batch tables
Validate Mapping http_status is in array

The constant was unused. Presumably it was once. I stringified the entries
because we store them as strings in the database.
Add validations to MappingsBatch

These are mostly duplicates of things on Mapping, because MappingsBatch is a
pseudo-mapping.
Create MappingsBatchEntries after creating batch
Be consistent when accessing the bulk_add object
Add MappingsBatch#process
Add Sidekiq

Had to bump mlanett-redis-lock because it had a conflicting dependency on redis.
Add basic worker for processing mappings
MappingsController uses background worker

There is no longer any feedback to the user, which is lame. I'll add that back
in next.