Don't allow wiki pages to have invalid names.
This incidentally means that you can't create wiki pages for pools. For
example, you can't create a wiki titled "pool:almost_heart-warming".
This is not a valid tag name, so it's not a valid wiki name either. This
was done in a handful of cases to translate Pixiv tags to Danbooru pools
(see: <https://danbooru.donmai.us/wiki_page_versions?search[title_like]=pool:*>)
Also fix it so that titles are normalized before validation, not before save.