Refactored and fixed artifact/catalog/show page

There was a lot of logic wrapped up in which links and messages to
display on an artifact page and tonight I discovered it wasn't working
as intended 100% of the time. I refactored all that logic out into a
helper. Should make things easier down the road too, if we ever need to
change this up.
Merge branch 'admin_interface' of https://github.com/WGBH/bostonlocaltv into admin_interface