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
We don't have a style guideline on this ([just checked][1]) but I'm accustomed to using the class name and the method under test in the exercise phase of the test. For me, it's clearer what the method is under test, but this may be personal preference (I can't find material supporting one side or the other).

Two questions:
1) Is it worth opening a Guides PR on this topic?
2) Does this violate the consistency of the repo (or the desired style)? If so, I'll revert the change.

[1]: https://github.com/thoughtbot/guides/tree/master/style#testing