Do a pass for unwanted characters

I already looked quite a bit for characters that would be better represented in a different way. The following command is what I'm currently using to list what's still questionable there:
```
git grep -P -n "[^\x00-\x7F αβλχΔΠπτΨ∑σµμ ©® ° ⁰¹²³⁵⁶⁷₁₂≥≈≠≤≥±−×÷√≪≫∗★ ←→↓↑↩︎ †§ ÄÈÁÉÍÓÚČŠàèáćéíóúýãñäëïöüůêôčěňřšžßç 😁 ⌥⌘⏎ –—─│┌┐└┘├█″′«»]"
```
However, even some of the characters used above may be best changed for easier editing. In particular, the math symbols can become MathJax.

Another aspect of this is dashes: - – — ─. Maybe we should either: A) allow Markdown to convert things like `--` to better dashes (does it?), or B) use `–` and `—` as appropriate.

Start by running the issue's git grep command across the repository and reviewing each reported character in context. Decide which symbols should remain, which should be represented with MathJax or entities, and how the dash cases should be handled; done means the questionable characters have an agreed, consistent representation.

Written by the indexing model from the issue text.