Fix regression in #4475. Fetch the commentary as html instead of
plaintext so that we don't lose links or other formatting.
Also fix it so that /jump.php redirect links are replaced with the
actual url.
* Move the source normalization logic out of the post model
and into individual sources' strategies.
* Rewrite normalization tests to be handled into each source's test,
and expand them significantly. Previously we were only testing
a very small subset of domains and variants.
* Fix up normalization for several sites.
* Normalize fav.me urls into normal deviantart urls.
* Rename `unique_id` to `tag_name`.
* Add `other_names` and `profile_urls` methods that sources can override
to provide extra names or urls when creating new artist entries.
Fix sources choosing the wrong strategy when the referer belongs to a
different site (for example, when uploading a twitter post with a pixiv
referer).
* Fix `match?` to only consider the main url, not the referer.
* Change `match?` to match against a list of domains given by the `domains` method.
* Change `match?` to an instance method.
e.g. If you were on an html work page on pixiv, clicked a link to a
different html work page on pixiv, and then clicked the bookmarklet,
then it used to fetch the source from the FIRST work you were on instead
of the second.
