Emoji support

Treat emoji replacements as atomic. Don't ever create a patch which features only part of an emoji.

emoji checking

if you replace 😂 with 😁, textpatcher's current behaviour is to attempt to replace only part of the emoji (since it stretches across several utf-8 code points.

This results in strange patches that break chainpad.

We've addressed this by throwing a crude error which warns the user if their action would break their current chainpad session, after which their tab will reload.