Support /mpv/ links for EHentai downloader

[fancydancers]

Adds support for /mpv/ links on the EHentai downloader plugin, I found it convenient when using with the Tsukihi extension so I didn't have to go back to the gallery view.

[Difegue]

Looks about right, any reason this is in Draft status? The Win integration tests are being a bit finicky but I don't think this is caused by the regex change here.

[fancydancers]

My main purpose for this is for use with Tsukihi but the current behavior of the patch is not ideal. After some testing I saw that when adding urls from mpv page the source: url will be the /mpv/ page rather than /g/ which I would consider the canonical source url and also if an archive already with a /g/ source url the urlfinder plugin won't find it from a /mpv/ page so it would prompt another download but then fail (or needlessly replace the duplicate archive depending on lanraragi settings). I'd like to fix those two issues first.