Markdown: require double tildes for strikethrough
Previously, either double or single tildes would work for strikethrough. However, this had some strange edge cases, such as attempting to strikethrough some words that contain a group reference. Since Tildes uses single tildes for other purposes, it should be much less ambiguous overall to just force double tildes for strikethrough like this. This will affect a small number of past posts, in both good and bad ways (getting rid of some strikethrough, but fixing some accidental ones).
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