Custom error for access to deleted post
Added custom error triggered by find_comment_id()
to make difference between deleted post and nonexisting one.
All API calls that use this SQL function are affected (unfortunately it is not yet used everywhere).
get_content
/get_content_replies
reimplemented as SQL functions
removed superfluous code from condenser_api
restored mute handling in get_content_replies
for consistency with get_content