Fix borders on datagrid age columns.
Fix borders on datagrid age columns.

The borders for age columns now override the default `border-bottom` and
`border-top` styles for datagrids to make the coloured border surround
the cells. The border for the oldest age cells has also been darkened to
make it more visible.

Testing Done:
Viewed age columns in the dashboard datagrid.

Reviewed at https://reviews.reviewboard.org/r/7525/

bkochendorfer/reviewboard,chipx86/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,davidt/reviewboard,bkochendorfer/reviewboard,beol/reviewboard,KnowNo/reviewboard,brennie/reviewboard,brennie/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,davidt/reviewboard,davidt/reviewboard,beol/reviewboard,beol/reviewboard,sgallagher/reviewboard,beol/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,brennie/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,brennie/reviewboard,bkochendorfer/reviewboard,KnowNo/reviewboard