Store path relative to Rails.root when tracking hits files

We use timestamped directories for releases in production. This
means that on every deploy the absolute filenames for hits files
change, even if the hits files in transition-stats and pre-transition-stats
haven't changed. All hits files are therefore re-imported after every
deploy, which is unnecessary and takes a long time.

To fix this, this stores the filename relative to Rails.root, thereby
excluding the part of the path which changes on deploy. @rgarner thought
he had a reason for not doing this originally, but can't remember it now.

It would be good to avoid this in production, but in preview it is
even more important at the moment. Preview is deployed to more frequently
than production, and the overnight data sync from production also
causes timestamp mismatches. We're trying to compare performance of
the hits import between MySQL and Postgres at the moment, and that
is very hard when the import mostly takes much longer than it needs to,
and longer than it would be in production.
Delete absolute filename ImportedHitsFiles

Each actual hits file will have many records in this table because the
filenames were being stored as absolute paths, including a timestamped
directory per deploy. This cleans those old records up so that we will
have only one row per real hits file with a relative path filename.

This will delete only rows with an absolute filename, in case someone
wants to rollback in dev and then migrate forwards through this again
without losing any relative-filename tracking.
Merge pull request #398 from alphagov/store_relative_filename_for_tracking_hits

Store relative filename for tracking hits