Remove unnecessary form inputs.
Fix broken possession-related forms.

The broken forms mentioned skills and spells. They should, instead, mention
possessions.
Clean up existing tests.

Rearrange code for greater logical coherence and readability.
Remove spurious references to "PUT".
Flesh out test case CharacterIdPossessionsTestCase

Test what happens when an HTTP POST request is made to URL
character/<id>/possessions/. Ensure that both valid and invalid POST requests
are handled correctly.