add indie game showcase to the list of interests

simple change
Minor quibble, but I'd slightly prefer the name of the config option to be ``indie_games`` and not ``indie_showcase`` and for the description to similarly just be ``Indie Games``.  Because we've got indie games not only in our big showcase but also in tabletop.

Note that if anyone has already registered with this checked, it'll be too late to change the variable name (without doing a database update) since the integer value is a hash of the option name.