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
> For me, it's clearer what the method is under test

The method under test would be present in the exercise phase. The object is also present, just not the class. The spec file denotes the actual class, so does the formatted spec output:
```
RepoActivator
  #activate
    when repo activation fails
      when Hound cannot be added to repo
        returns false
```
Moving the initialization of the object out of the exercise phase, makes it more focused just to the method described.