Put transition date on analytics graph

* Show a line on the analytics graph on the date at which the site
mappings went live
* Only show a line when site is live and the graph spans the date of
transition
Merge pull request #260 from alphagov/transition-date-on-graph

Put transition date on analytics graph
Tidy up bulk add success messages

* When all are updated, don’t mention “0 created”
* When both updating and creating, don’t show a broken tagged message
(“All .”)
* When tagging, conditionally append “All “ to the start of the
tagged_with string
* When only “1 mapping updated”, remove the full stop as the message is
effectively a title.
Merge pull request #261 from alphagov/fix-broken-modal-title

Tidy up bulk add success messages
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.