Dispatch backend translations into own modules

Today, backend translations are "aggregated" into openimis-be_py module.
When adding a new module, translations have to be 'merged' manually into the 'assembly' project.

Setup a 'load' mechanism where translations would be picked from:

• one specific module (a 'language pack')

• the module's provided (default) translation

• ... the central (assembly project) translation (for shared translations between modules)