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).