Better handle blocking a default app

[cassidyjames]

Right now it's possible to block a default app (i.e. the web browser), which gets funky when doing things throw a "you're not allowed to use this app" dialog. We should probably do one or more of the following:

1. Warn/inform the user when they're blocking a default app
2. Disallow blocking a default app
3. Prompt to change the default app when blocking a default app

I think I'm against 2 as that would restrict how it can be used too much. 1 or 3 (or some combo) could probably work.