Support changing instance page titles

Now admins can choose a title for their About and Privacy pages; now
editable through the instance page editor.

This adds title and content_type fields to the appcontent table,
requiring a migration by running writefreely --migrate

The content_type field specifies that items we're currently storing in
this table are all "page"s; queries for fetching these have been updated
to filter for this type. In the future, this field will be used to
indicate when an item is a stylesheet (ref T563) or other supported
type.

Ref T566