Modernize Youtube embeds and exempt from Bleach cleaning, abandon Imgur embeds

[HebaruSan]

Problem

https://spacedock.info/markdown says:

You can also embed YouTube videos and Imgur albums by wrapping the URL in brackets (specific to SpaceDock):

[[https://www.youtube.com/watch?v=T4rfHAqs9EI]]

However, if you try to use this currently, it just renders as an escaped inline <iframe>:

https://spacedock.info/mod/141/scatterer

Cause

#336 added a Bleach sanitization pass to our Markdown rendering, which only allows certain tags and attributes to be included in the rendered HTML. Currently iframe is not on the allowed list, and the above embedding functionality works by generating an iframe element.

Changes

Now iframe is allowed, but we apply extra validation to it. Only the attributes generated by SD's embed code are allowed, and for src specifically, only values beginning with the URL prefixes we use are allowed. If someone tries to use an iframe with an unauthorized URL, it will just show up as empty:

Fixes #383.
Fixes #178.

Side fixes

The embedding functionality was using a deprecated API (assigning a Pattern directly to a member of md.inlinePatterns) and is now updated to the newer register API. As part of this the KerbDown-related classes are refactored somewhat to put certain values and functions in the classes where they belong.

If you click "SHARE" on Youtube, it presents you with a link in the format https://youtu.be/video_id. Now we support embedding this format.

Imgur's embedding hasn't worked in a long time, and shows no signs of ever being fixed. Now it's removed from the code and from the markdown help page.

[DasSkelett]

Thanks a lot! Works marvelously. And doesn't compromise security, for what I can say with my very limited knowledge in that area.