Add validations to HerokuApplication model

[hiattp]

From #1:

It'd probably make sense to just add some defensive validations to ensure that any application we create has a name, oauth token, grace period defined.

Also would be kinda good to be able to test that the key works (ie. can authenticate) before it tries to actually trigger a restart.

We should:

1. Add a db constraint to the name and grace period fields to ensure presence
2. Ensure we get an exception thrown if the HerokuAppRestarter fails (e.g. due to a bad/missing oauth token). That way we'll be alerted if this isn't working both initially and on an ongoing basis