Rationalise summary part of the merge screen.

Rationalise summary part of the merge screen.

This does 3 things
1) reduces contact get queries from 3 to 1 since the 2 in the tpl layer are removed
2) adds created date to the merge summary information
3) makes it easier for extensions to replace/ extend this data (although core makes no
commitment to the tpl remaining unchanged so extension writers need to carry that risk)