If the setting is turned on, then the user will receive information
about the subject: if it was deleted or requires some special access to
a group (only if the group is public). Otherwise, the user will receive
a generic #404 error message. For now, this change affects only the
topics and categories controller.
This commit also tries to refactor some of the code related to error
handling. To make error pages more consistent (design-wise), the actual
error page will be rendered server-side.
This feature can be enabled by choosing a destination for the
`shared drafts category` site setting.
* Staff members can create shared drafts, choosing a destination
category for the topic when it is published.
* Shared Drafts can be viewed in their category, or above the
topic list for the destination category where it will end up.
* When the shared draft is ready, it can be published to the
appropriate category by clicking a button on the topic view.
* When published, Drafts change their timestamps to the current
time, and any edits to the original post are removed.
