Makes these tag names invalid when tagging posts:
* Blank names (e.g. ___).
* Non-ASCII characters (Japanese text).
* Non-printable characters (e.g. control characters < 0x20).
* Leading or trailing underscores, or consecutive underscores. Reason:
`_foo__bar_` and `foo_bar` both render as `foo bar` in the tag list.
* Leading `-` or `~`, or '*' (-foo, ~foo, foo*bar). Previously
these were silently stripped, but that meant tagging a post with e.g.
`*-foo` tagged the post with the invalid tag `-foo`.
* Tag type prefixes (e.g. `character:character:hatsune_miku` no longer creates
the literal tag `character:hatsune_miku`).
* Metatags (order:score, user:evazion, etc).
853ddc7cda