Fixes an exception when attempting to upload a Pixiv image from a
deleted work. The download strategy tries to fetch the source data in
the course of rewriting the URL, which fails if the work has been
deleted from Pixiv.
Raise a BadIDError and leave the URL as-is (don't rewrite it).
Fixes an "incompatible character encodings: UTF-8 and ASCII-8BIT"
exception when uploading a deleted Pixiv post.
Caused by the Pixiv API error message not having the encoding set to UTF-8.
* Only suggest the Danbooru tag with the same name if there is no
matching wiki other name. Example: if we have the Pixiv tag `Fate` and
the Danbooru tag `fate_(series)` with other name `fate`, suggest that,
not the Danbooru tag `fate`.
* Don't suggest tags that are empty or whose wiki is deleted.
* Only split tags on "/" if there are no other matches, and only for Pixiv.
* For Pixiv, only include traditional media tags in tag list, not digital media (Photoshop, SAI).
* Add some tests.
