Importer: additionally add all device configs to local git repo

[tpurschke]

• add local git repo on backend server
• do this in addition to DB Import
• do not write the full (raw) config to db table import_full_config but to git
• also add Import id to git
• might also be used to add routing capabilities later
• git diff saving function should make sure not to collect too much data
• do not save any unchanged configs
• do not save binaries
• to be used as a minimal starting point for all (future) import modules

[NilsPur]

@tpurschke phase2 ?