Feature improve plugginability

[Crivella]

Plugins can now be used to also add models to the database via the following entrypoints:

• ocr_translate.box_data
• ocr_translate.ocr_data
• ocr_translate.tsl_data

The entrypoint should point to a dict with the info to create the model.
See init of plugins for example (care that box/ocr/tsl may need to define different keys).

Information about model-specific language codes is now encoded into an iso1_map field of the model.

• Before new models with custom codes in a plugin would require to also edit the main repo and adding a new column to languages in the database.
• Now the plugin can set the lang_code to whatever is closest to the model codes, and overwrite what does not match using iso1_map, by mapping iso-639-1 codes to the model-specific ones.

Tag only without release as the changes still requires plugins to be baked in with the installer (they cannot be dynamically added without an hack-ish solution).