Bug-fixes in inject / calc_sums / verify

Adjusted pathnames and build rules to run in gitlab instead of Jenkins

• Dropped python2
• Replaced database table backups with .secondshot-manifests (kept with each saveset) for efficiency

To upgrade, invoke secondshot --action=schema-update before running backups. This will drop the backups table; mysqldump your database first.

Now supports 64-bit index values for the tables that store file metadata.

Now supports python3.

This adds a config table to the database, way more unit-test coverage, and better configurability.