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).
Let's go ahead and wire this up in `wiki.urls`. When it's merged, add a comment to [the bug](https://bugzilla.mozilla.org/show_bug.cgi?id=811007) describing the URL interface for the front-end part.