Implement responding to follow requests.

Implement responding to follow requests.

If a user has requested to follow you, you get an extra menu item in
the options menu for that user, where you can accept or reject the
request, or leave it unresponded-to for another time.

To make this optional menu item less painful to implement, I made the
MenuKeypressLineGeneral trait also apply to Option<T> for any inner T
that also implements it. So I can do all the routine admin like
ensure_widths without writing three annoying 'if let Some(...)'.

(But that doesn't let me get_value() on an optional CyclingMenuLine -
that would be silly! For retrieving the value, or cycling the line,
you still need to check if it's actually there.)