The dashboard should generally only check for permissions not flags.

refs #127

The AddShowButton was not rendered when the shows weren’t loaded, but
the only requirement for it to render should be if the user is a
superuser.

refs #89

We’ve decided to move the show selector into the navbar because the
selected show is the primary state of the dashboard from which almost
all other state is derived.

Apart from that we now have support for filtering the list based on
active and inactive shows (and possible other criteria in the future) as
it was suggested in #122.