uistyle: Ignore format code for Monospace

Since the current rendering engine does not easily support switching
fonts in the middle of a message, we can't currently support the
Monospace format code as specified in [1].
However, to avoid weird artifacts when other clients send this, ignore
the format code for now.

[1] https://modern.ircdocs.horse/formatting.html#monospace

Closes GH-342.