Use identifier to reference sources

[nesnoj]

There's a problem with the source ids in meta app:

When I create sources locally, they get an auto id (postgres serial) in my local DB which can be equal to an existing (different) one in the WAM DB.
We reference sources by those ids (example) which must be manually changed on deployment to match the WAM DB ones.

Current state
In the current state, the WAM's meta tables must dumped and restored locally every time a source is added in the WAM (never restore the locally amended meta tables on the WAM as sources from other apps are is likely to get overwritten!).

Future
In rl-institut/WAM#86 I proposed an additional column holding an unique identifier such as "bgr_mun_2018" which can be clearly referenced to. StEmp ABW must be changed accordingly in MasterDetailView as soon as rl-institut/WAM#86 is done.