Rename SUPPORTED_STATUSES to be http-specific
Add a 'status' column to the three mappings tables

This column stores the mapping status as a word string rather than as
an HTTP status code string. There isn't an appropriate HTTP status code
to use for unresolved mappings, so this is the cleanest way to store
users' decisions about mappings in the long run.

The new column has the same null constraints as the existing
http_status column on that table in each case.

This commit also adds validations for Mapping.status and sets the status
from http_status before validation. The status is not yet being set
correctly everywhere, but all the existing tests pass.
Set mapping status when importing from Redirector
Add a test to document import of '418' mappings

We currently have 198 mappings in production with an http_status of
418, which Redirector used for 'pending content', to signify that
content was coming but was not yet published, so couldn't yet be
redirected to. Bouncer serves 404 for these.
Only set status if http_status is a valid type

This means that the standard shoulda matcher for testing inclusion
validation works, because otherwise it wasn't able to set an invalid
value without it being changed by the callback before validation.
Add a one-off rake task to set status for existing mappings
Preserve '418' http_status as 'pending_content' status

This isn't a valid mapping type, but we already have 198 mappings
with http_status '418' in the production database because they
were imported from Redirector. We should not lose this information
about these mappings when switching to using the new status column.