Unify unicode escaping behavior

[eliotwrobson]

See title, there are a number of elements (pl-symbolic-input, pl-string-input, possibly pl-units-input) that accept a variety of different characters, so weird unicode could slip in somewhere. These elements have different ways of displaying parsed vs raw student input to mitigate confusion by students. Find a way of unifying the display and behavior across elements.

This hits a lot of elements, so probably shouldn't change until summer.

Initial discussion is here: #9438 (comment)