Add concept of "available hours" to users

Now that Launchpad has maps and timezones are shown in different ways, it would be useful if we would allow users to set in what hours they're available (ie, some people may be available during the day, some during the night).
Quoting Mark:
"At Canonical we have the idea of "core work hours". We could generalize that to "hours I am contactable", ask people to provide that, and show the icons differently on the team map."

This would also work well with the Launchpad team :)