Allow enforcement of group shows #105
Description
Add a new permission that allows users to submit solo shows, and automatically removes solo shows from the schedule builder if they're submitted in violation of this permission.
Context: Removing permissions to host a solo show is a possible compliance sanction.
Notes to developers: This will require a significant rewrite of the show applications handling, as the spatie/laravel-permission package can't easily handle "anti"-permissions like this. By default the permission should be available to accounts, but it should be easily removable in case this sanction needs to be issued.