at -> in for consistency of location reference

FAQ says "in $location", location is currently "Cardiff", so lets have
the front page say "in" too for consistency, and keep the $venue as "at"
"in $date" -> "on $date", if $date

"in January" / "on 24th January"

This is still far from perfect, it doesn't handle the case where we want
to say "in $month", but my argument is that if we only have month
resolution, that might not be confirmed, so lets say a confirmed date or
nothing at all.

(and by "argument" I really mean "excuse for not covering the month-only
case" ;))
Added Cardiff details
Merge pull request #122 from nhshackday/feature/cardiff-frontpage

Advertise Cardiff (with correct prepositions)