Corrections related to bridge.get_discussion + some cleanup

Andrzej Lisakrequested to merge
abw_get_discussion_mutes into develop

find_account_id changed to be similar to find_comment_id (other routines like that will likely follow) - True means the check will be performed, but not when account is empty (that needs to be blocked/allowed in python code)

get_discussion SQL function renamed to bridge_get_discussion for consistency

added validation of observer parameter in bridge.get_discussion

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