Content directory smart contracts: Languages, categories and licenses

If we'd only rely on the initial idea described in https://github.com/Joystream/joystream/issues/1816, then in order to update the set of avilable languages / categories / licenses we would need to update the metadata json schema that includes them (ie. in form of json-schema enum type), create a PR, publish new version of content directory lib and CLI and possibly update and re-deploy all other projects that rely on the previous schema (ie. query node and possibly Atlas once it includes uploading / channel management)

The easiest way to avoid that would be have some representation of categories and languages in smart contracts instead. This seems to be the only way to handle "choice type" video/channel properties for which we want curators / lead to be able to add new possible choices without the need to change any actual code (so perhaps that would also include licenses?)

In that case each language, category and license would have it's own distinct metadata (provided as json string) and the contracts would expose methods to add / update / remove entities, executable by the lead.

Start with the initial idea in issue 1816, then inspect the content directory metadata JSON schema, library, and CLI described here. Consider how languages, categories, and licenses would be represented in contracts and how the query node and Atlas might depend on the schema. Done means the scope defines contract methods for lead-controlled add, update, and removal without requiring schema or code releases.