U4-5290 - allow for retrieval of a tag's count as well as it's text

This is available from the obsoleted Tags API (umbraco.cms.businesslogic.Tags.Tag.GetTags), but currently isn't in the newer TagsService. 

To implement this I've added a new property called NodeCount on ITag/Tag, and an optional flag called "withCount" to tag retrieval methods on TagService.  If passed, the SQL is modified to do a GROUP BY/COUNT rather than a DISTINCT.  Actually all the tables necessary were already joined, so I wouldn't expect much of a performance issue from retrieving the count.
Hrm, but that change fixes the tag repo tests, but now the tag service test is failing with the column attribute removal. Will debug into this.