WIP: change default charset to utf-8

UTF-8 is a sane default for many reasons.

First, it's a superset of ASCII, which means that existing strings that were covered by the previous settings will still work.

Second, it is broadly used on the web. As a native french speaker, I have sen the transition from latin-1 (ISO-8859-1) happen about a decade ago and I am not aware of anyone still using latin-1 voluntarily.

Third, defaulting to US-ASCII doesn't help in any way. It brings trouble to the vast majority of users who do need non-ASCII characters in their daily lives (see for example bug #542).

Finally, I believe most email systems out there are now 8-bit clean. That might be a broad statement, but I think it's reality these days. I would also be willing to say those who are not 8-bit clean deserve the pain they will get.

Closes: #542

Note that this MR was based on !444 (merged) to make sure CI actually passes, so that should be merged before this.