feedback: Restrict message length to 4096 characters

[armcknight]

Description

This will require a character counter underneath the text field, and new validation/error reporting logic

Need to include testing against our backend what happens with unicode precomposed characters: https://en.wikipedia.org/wiki/Precomposed_character#:~:text=A%20precomposed%20character%20(alternatively%20composite,letter%20e%20with%20acute%20accent).