Allow starting a process on non-folder elements without an image

Sentry Issue: ARKINDEX-FRONTEND-AXY

AxiosError: Request failed with status code 400
  at AxiosError (./node_modules/axios/lib/core/AxiosError.js:22:18)
  at settle (./node_modules/axios/lib/core/settle.js:19:12)
  at onloadend (./node_modules/axios/lib/adapters/xhr.js:100:7)

@schneider-y tried to start a process on this element in demo, which is not of a folder type and does not have an image or polygon (commonly referred to as an act in Himanis-speak). CreateElementsWorkflow however does not allow this, requiring an element to either be a folder or have an image and polygon.

This requirement was implemented here, as a direct translation of a Himanis requirement during the Type Revolution. This requirement was added in this commit after this discussion. At the time, the init_elements task could only support either pages or volumes, aka elements with images, or folder elements.

This backend restriction was made unnecessary in October 2019, in teklia/arkindex/tasks!47, when init_elements started supporting any element type. We can remove this and allow running a process on anything. Note that this restriction is already not applied anyway when selecting the element and then creating a process on the selection.