Refactor class CampaignId

Remove class method `CampaignId.get_campaign_or_404` and create function
`_get_model_object_or_404`. Add module `views` to test suite (the function has a
doctest).
Add campaign-related views.

Add the following view classes:
* CampaignIdUpdateForm (for path campaign/<id>/update-form/)
* CampaignIdDeleteForm (for path campaign/<id>/delete-form/)

Also add HTML templates which those classes use.
Add campaign-related paths to module `urls`

Add the following paths:
* gurps_manager/<id>/update-form/
* gurps_manager/<id>/delete-form/
Write tests for campaign-related paths.

Write tests for the following paths:
* campaign/<id>/update-form/
* campaign/<id>/delete-form/