Improve document schedule

[Migaroez]

Description

The Management API currently supports publishing OR scheduling of documents, but there is no way to cancel schedules or clear them.

This PR aims to rectify this shortcoming while still maintaining the API model/endpoint and the spirit of the endpoint.

Details

With this PR, it should be possible to

• publish one or more cultures
• schedule (publish and or unpublish) one or more cultures
• update the schedule of one more cultures
• clear the schedule for a given culture or cultures
• combine any of the 4 actions above across multiple cultures (each action can not be combined within one culture)
• read the schedules per culture when requesting the document

Notes

A few things came up while making the necessary changes, please keep these in mind when reviewing/testing this PR

• The invariant culture string was mixed between "" and "*", where it makes sense (and there was no sign it would cause any issues) this has been aligned to "*" and placed in a constant for clarity
• Regarding full truth vs partial truth: It is very hard, maybe even impossible to combine full context truth (document) vs sub context truth (per culture) in one model. To avoid breaking the endpoint signature and with the idea of allowing editors to only send the data they are working on (one or more cultures) to be updated in the future, this endpoint supports just that.

Testing

You can look for inspiration in the added integration tests, but please try to think outside the box.

[bergmania]

Just need to fix conflicts but otherwise I think it looks good