Fixes and code cleanup in bridge API

bridge API get_post/get_relationship_between_accounts separated as SQL functions (checks supplemented in the same time and errors unified with other calls)

[WIP] bridge API routine load_posts_keyed simplified (especially with regard to communities) - on the road to being completely removed

[Fix] pinned community posts no longer can show up twice in get_ranked_posts calls

[Ign] append_statistics_to_post does not need to be async

[Ign] removed some unused code from bridge API

[Ign] get_number_of_unreaded_notifications renamed (unreaded => unread)

[Ign] tabs removed from places where they crept in

[Ign] 0 used as neutral value instead of -1 for page in account_notifications/post_notifications