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
- remain backwards compatible on method-level
- fix FakeResultWrapper::fetchRow, which (contrary to ResultWrapper::fetchRow) didn't always return an array

Change-Id: I74c3784d6d93b992d72f2db24cc29f30e458c1e3