Allow POSTing and PUTting section images and sections separately instead of inlined

[Rikuoja]

Currently, the only way to edit an existing Hearing is to PUT the whole shebang with all the images and sections inlined. This is very susceptible to effects such as the UI state accidentally misplacing some sections, which are then deleted as a result even if the user did not mean to edit them at all.

Therefore, the section and image endpoints should accept sections and images directly, so the UI may POST/PUT a section or image whenever one is added or edited to a Hearing.

Deleting sections or images should not be allowed inlined in the hearing endpoint.