Fix bug 811007: Provide a way for users to flag pages for deletion.

This repurposes the existing contentflagging app, adding:

1. A new set of wiki-specific flag reasons

2. A wiki-specific form and view for flagging content.

3. A consolidated view which provides a list of all flagged content,
   organized by content type.

This will still require templating/UI work, to display the flagging
interface in wiki pages (much as it's already displayed on demos), and
for the flag list in (3) above (to ensure tests pass, a blank template
is provided in this commit).
It's fetching only those which have the given status. Which, by default, is going to be only those which are flagged but not yet handled by someone (since that changes the status). So it should be a safely-small QuerySet.