Introduce a new hook that allows extensions to add to My Contributions

Introduce a new hook that allows extensions to add to My Contributions

Meanwhile also:
- refactored reallyDoQuery in Pager.php, to make outside intervention possible
- extend reallyDoQuery in SpecialContributions.php, adding in the hook and composing the final results array based on the combined results of all queries added through the hook

Change-Id: Ifd63cdf1950f914a3fba1757a905385aca311eda