Change Hearing editor logic to POST/PUT Hearings, Sections and Images separately

[Rikuoja]

Currently, the whole Hearing with all sections and images inlined are POSTed/PUTted together. This is very risky when editing complex Hearings, as evidenced by e.g. accidentally deleting sections from live Hearings. Also, saving changes in large hearings gets rather slow.

Therefore, after backend support City-of-Helsinki/kerrokantasi#304 is implemented, the editor logic should be changes so that a draft Hearing is POSTed when moving to the contents part of the editor, and sections are POSTed one by one whenever a new section is added to the Hearing. The same when an image is added to any section.

Then, also editing sections and images may be done one by one with PUT, and trying to DELETE anything from the backend should come with a warning to the user, so no accidental deletions occur.