Replace all '.uncached' calls by '.objects.no_cache()' (bug 913010)

https://bugzilla.mozilla.org/show_bug.cgi?id=913010
https://bugzilla.mozilla.org/show_bug.cgi?id=912178

This completely removes the `uncached` generic manager and replaces it with calling `.objects.no_cache()`. This fixes at least 2 weird issues we've been having: non-webapps being returned and deleted stuff being returned when calling `Webapp.uncached`.

I probably could have kept `uncached` for the other models, but we don't really need to have multiple ways to do the same thing, and it's really annoying to make it work properly when you throw inheritance and soft deletion into the mix anyway. 

**Work In Progress!** We probably want to wait until after monday tag has been created to merge this: it's best to let this live on -dev a few days to be sure. I also want to add a couple additional tests.