Properly use offliner definition version

[benoit74]

This is a follow-up to openzim/zimfarm#1266

This PR introduced the notion of offliner definition to correctly model that scraper flags are changing over scraper releases.

We need to adapt zimit-frontend to:

• store somewhere which zimit offliner definition version is currently in use
• properly set this version to use when creating a schedule
• properly retrieve the definition of current version to display flags

[benoit74]

@elfkuzco I feel like you can propose a PR here, and I will review. Is it ok for you?

[elfkuzco]

Yes