Secondshot

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.