Small fixes and mock data for observer tests in get_ranked_posts

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

tests/tests_api

-Subproject commit 204e7fa2128efdfd5c197bc2ab76fc37e414cc88
+Subproject commit 10288df4f5217ee9c38b5fb8e8c0b916500a7df9