find_tag_id/find_category_id moved to utility_functions.sql and changed so they follow example of find_comment_id with their error handling

added extra promotion and new ignored accounts in mock data for observer tests

observer in get_ranked_posts is now validated (it was actually validated already, but only after main query, now it happens as early as possible)

fixed problem with default value of limit in get_ranked_posts (when explicit null was passed); also all string params now are empty instead of None