Allow 'For the 1st Time' and 'For the Most Recent Time' query logic

[ndobb]

In the previous UW-specific implementation of Leaf, Leaf allowed users to query for the first or final time a concept was found for a patient (i.e. the 'index' event):

This is a feature which is requested or asked about somewhat frequently, and should probably be re-added in the new version.

Note: re-adding this should probably be done after the SQL compiler is refactored.