Fixes a bug from 9a3824a. When an artist entry is saved, `ArtistUrl.normalize`
is called on every URL, which calls `Sources::Site.new(url)`. This
raised NoStrategyError when an artist entry contained URLs that weren't
recognized by any strategy.
This also caused `Fetch source data` to fail in certain cases when it
attempted to find the artist.
* Convert Tumblr commentary to DText.
* Strip extraneous whitespace in links and blockquotes.
* Add newlines after block elements to ensure they're separated from
subsequent blocks.
