Add tests for class views.CampaignId

Write tests for previously untouched code paths. Rewrite existing docstrings.
Add human-readable error messages.

When an HTTP 403 (HTTP Forbidden) mesage is returned to a user, also return a
human-readable error message such as "Error: you do not have the rights to view
this campaign."
Add tests for campaign update and delete forms

Ensure that a user can only fetch those forms if they have the necessary rights.
Also, make docstrings in module `test_views` more consistent.
Add tests for updating skills and possessions

Test whether a user can update a character's skills or possessions if they have
insufficent rights.
Update unit tests for view CharacterIdSpells

Also: remove unnecessary field from HTML form field used to update a character's
spells.
Fix unit test in CharacterIdPossessionsTestCase
Rename classes.

Rename classes in module `views` for the sake of enforcing consistency.
Add unit tests for view CharacterIdTraits
Add unit tests for character hit locations.
Rename classes.

Rename classes in module `views` for the sake of enforcing consistency.
Fix error messages
Add "breadcrumb" nav elements to HTML templates
Add unit tests for view CampaignIdItems
Add unit tests for view CampaignIdSpells
Remove unnecessary backslashes
Add unit tests for class CampaignTable
Add unit tests for class CharacterTable

Also: remove a useless doctest. (It was not being run.)
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