EnhancedChangesList: Don't render headings for empty days

EnhancedChangesList: Don't render headings for empty days

EnhancedChangesList renders an <h4> for every day, but it
does so before it starts rendering the changes for that day.
If all of the changes for a different day fail to render
(due to permissions issues, extension hooks, or whatever)
this would result in an empty heading.

Instead, render the heading after formatting is complete,
so that if all changes for a given day are dropped,
the heading is also dropped.

Bug: T171078
Change-Id: I8a0c6cbd679976d18d2c2e6e9ac972fb7b294a42