[ABW]: last_month() moved to common to be shared across APIs (it should also...

[ABW]: last_month() moved to common to be shared across APIs (it should also be changed to take month from head block but it was not done yet)
changed checks on numerical values - now empty string is treated as if parameter was skipped; also invalid cast to int no longer generates Server Error, but 'invalid parameters'
[Fix] restored older version of list_top_communities, since most recent change was apparently experimental and introduced hardcoded community id into query; also fixed check on limit there
[Fix] fixed list_pop_communities; it was apparently never used since it crashed and its code did not change since inception
[Fix] added checks on min_score and post_id to post_notifications so they can generate meaningful errors (similar change should be done in account_notifications)
[Fix] post_notifications again reacts correctly on missing post specified in its call parameters (similar change needed in account_notifications for account param)
tabs removed from schema.py