DI: global $wgCampaignsDI, improve tests and doc

Minor improvements to dependency injection system as suggested in
code review here [1] and here [2].

[1] https://gerrit.wikimedia.org/r/#/c/116153/9/includes/setup/Setup.php
[2] https://gerrit.wikimedia.org/r/#/c/116221/21/Campaigns.php

Change-Id: I055fe9cc75812b06691af36ab1e8bdf5e9c202b1