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
There's a lot of conditional logic in the `RepoActivator`, so I need to pass back a `true` [here] to ensure that `RepoActivator#activate` returns a `true`. (https://github.com/thoughtbot/hound/pull/461/files#diff-6a50d8a7e1182010baf5ac2ba63a5a1cR10)

Let me know if this seems unnecessary.

[here]: https://github.com/thoughtbot/hound/pull/461/files#diff-aaa1fe70069044ed5ccfceeaf7a87c8cR60