Exporting comments for communities

[jace]

Title
Exporting comments for communities

Short, concise description of the idea
The export_comments.bml interface only works for individual journals. It should take a `usejournal` parameter to export comments for communities.

Full description of the idea
LiveJournal's flat and xml-rpc interfaces take a `usejournal` parameter to indicate that the client wants to work with a specific community that the user has administration rights for. This can be used to download all posts made to a community, even by other users. There is currently no way to do the same when exporting comments since the `usejournal` parameter is ignored there. The export_comments.bml page could do with this feature.
An ordered list of benefits

• Users who use personal communities to classify their posts (for example: evan_tech and jacemobile) will be able to backup comments.
• This closes a gap in the data backup API.

An ordered list of problems/issues involved

• None.