Clean up mentions of US-ASCII

[annevk]

Whenever US-ASCII is mentioned there is a bug, since that encoding does not exist per the Encoding standard.

[annevk]

Most of the US-ASCII usage which can be found in the forms section can probably be replaced by language that uses UTF-8 algorithms instead, but I suspect that the forms language needs updating anyway. I don't really trust those algorithms to be completely correct.

[domenic]

There are only 9 of these, FYI. Might be a good bug to knock out ;)

[annevk]

This seems to require rewriting most of form submission. Started with #1325 and #1326.