Make the generated number more visible.

I believe my bad eyes and bad light conditions of my office are not that
unique, so making the generated keys more visible could be useful.

And yes, 'font-family: monospace' is rather brutal thing, but I believe
it really helps
(http://www.joelonsoftware.com/uibook/chapters/fog0000000063.html)

Actually, after thinking about it and looking at how are numbers
presented on the Android Google Authenticatior
(https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2)
it seems to me that we can go all the way to xx-large.

Signed-off-by: Matěj Cepl <mcepl@redhat.com>