Allow API users to cancel a scheduling job

[Flix6x]

This feature would enable users to cancel a potentially large scheduling job in case they no longer have a need for its results (e.g. they got new information they want to have taken into account, and intend to trigger a new schedule).

I'd suggest the following logic under [DELETE] sensors/(id)/schedules/trigger with a single JSON argument (the job's uuid):

• A job in the scheduling queue could be cancelled like this.
• A job already in execution may possibly be cancelled like this.

[nhoening]

If #574 happens, we might also have to remove/update the latest schedule ID / flex model, unless this is only written after a job is computed.

[nhoening]

Isn't this in-built RQ solution better for canceling an ongoing job?

[Flix6x]

Isn't this in-built RQ solution better for canceling an ongoing job?

I missed that one. It is better.