Insert a finger-print into a link that is based on the last modified date.
This way we can use far future cache headers on images/assets, and simply change the filename for the
asset when the asset has changed and we want browsers & proxies to take up the new asset from the server.
Appends the date as a string directly before the file extension.
The original link with the date based finger-print inserted, null if the passed link is null or the original link if lastModified is null.