Flesh out test case CharacterIdSkillsTestCase

In particular, test what happens when an HTTP POST request is made to URL
character/<id>/skills/.
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.
Add and refactor tests for view `Login`

`coverage` reports that all code paths in view `Login` are covered.
Add a test for view CampaignId

Ensure a user cannot view a campaign they do not have the rights to view.
Update css

Use a tiled image for the site background. Make the body background white and
slightly transparent, change the color of the header border, and add a border to
the footer.
Background image foolery