Adding the ability to call a local stylesheet (Bug 1346926)

This will be called last if it exists so that one can overrule any
styles they don't like without having to edit a theme or create their
own theme.

For example: don't like how the blockquote or hr tag is styled then
add your own local sheet to handle it.

See /local/lib.php for an example.

Change-Id: I5ab1e9954d623697b3416057a66c9830c9fa7587
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Adding the ability to call a local stylesheet (Bug 1346926)"