Manage available pipeline & associated steps in backend instead of frontend

[raphaelvignes]

Currently when we want to manage which button/step to display in frontend we need to declare available step of a translator in
src/lib/translators/.
Perhaps, we could manage this list directly from the steps listed in:
server/weaverbird/backends/xxxx/steps/__init__.py ?

[davinov]

Yes completly!
This is one of the tasks suggested in #912 : "rewrite the supported/unsupported steps logic in the UI part to remove the dependency to any translators"
This could be declared in a JSON file shared to the UI. Even better if we manage to generate this file from a convention on how the backends are written ^^