search for tags

Currently this is done with a regular expression.  Tags should
ultimately be stored separately, so no regular expression is required.
filter hidden posts faster
use faster post.local? instead of post.member.account
refresh_posts: filter posts on query flags
refresh_posts: filter posts on visibility, via, from
refresh_posts: run phrase and tag regexps in db layer
refresh_posts: speed up final ruby filtering
remove query_components and query_parts
tests: parsed_query keeps hash in front of tags
add TODO comment