[weasyl] Fix favourites extractor and add support for the readable URL format

[Deer-Spangle]

When you click favourites on Weasyl, it tends to go to the clean URL format:
https://www.weasyl.com/favorites/furoferre
Rather than the user ID format.

The user ID format still works, but you click "more submissions" to get there:
https://www.weasyl.com/favorites?userid=184616&feature=submit

This change allows the extractor to use the clean URL format, as well as the user ID format. It just uses the first to get the user ID for the second.

I did notice btw that some accounts have character favourites and things, alongside submission favourites, for example this account just has character favourites:
https://www.weasyl.com/favorites/jaggy
and those are listed with feature=char:
https://www.weasyl.com/favorites?userid=12345&feature=char
But the weasyl extractors don't have support for characters anyway. But thought it notable

[Deer-Spangle]

The last failing test build, for python 3.5, seems related more to yt-dlp, than to any change on my side here.

Also, I notice that weasyl extractor downloads favourites into files such as:

weasyl/{uploader}/Favorites/{id} {title}.png

Which means that basically ever favourite ends up in a different directory.
But similar other extractors, like the Furaffinity extractor, extract into:

weasyl/{user}/Favorites/{id} {title}.png

Is this a mistake? Can I change that?

[Deer-Spangle]

In fact, given the line:

submission["user"] = owner_login

It looks like it's just a bug that it doesn't download to one directory, so I'll just change that too

[Deer-Spangle]

Oh, I hadn't noticed, but the Weasyl fav downloader where it will go into an infinite loop on the final page rather than finishing cleanly. (And then it sends lots and lots of useless requests to Weasyl, oops.)

This is because the "&nextid=" appears in every page, in the sfw link at the top of the page, rather than finding the actual next button.

I've added a fix for that now also