entitySet: do not ignore licenseId set by editor

[elbotho]

What is wrong?

Currently the API always uses the default license when creating new entities:
https://github.com/serlo/api.serlo.org/blob/staging/packages/server/src/schema/uuid/abstract-entity/entity-set-handler.ts#L151

How should it behave?

It would be nice if the API would use the licenseId set in the editor (in the serlo-template-plugins).

Proposed Solution

We need to parse the entity content (outermost plugin) and retrieve and use the licenseId if there is one. Otherwise falling back to default like before is fine.

[elbotho]

(serlo/frontend#3501)