Create a cached version of the talk counter service that returns a
cached result of the talk count for that day.
This uses the Decorator design pattern to decorate the existing
`TalkCounter` service and works as they both implement the same
`TalkCounterInterface`.
Technically, if a talk is due to be given today, it hasn't been
presented yet and shouldn't be included within the talk count.
This commit changes the comparison to match on talks that were given
yesterday instead of today.
Oliver Davies