Check repos for valid information when activating

* Don't activate repos where either `private` or `in_organization` is nil
* Return `true` when pushing to a job queue

Minor:
* Move enqueueing of `OrgInvitationJob` into the activator
* Use instance method for activator to cut down on redundant argument-passing
* Sort private methods into order in which they are called
* Use named parameters
* Use `.fetch` for retrieving hash values (raises rather than `nil`s)
* Avoid ternary operators

https://trello.com/c/fABEW1eM/391-don-t-allow-repo-activation-if-repo-is-missing-privacy-or-organization-information
The issue with returning true is that it doesn't communicate why it returns that. It's not a predicate method, so it's a little surprising.

Do we need to return true from here? Won't `Resque.enqueue(job_class, *args)` be truthy?