Add a mechanism for documenting dead links

[ctrueden]

From a conversation on Gitter:

@velissarious wrote:

Why not link to https://archive.org/web/ based on the date of the last edit and the dead link?

For example this is the page with the instructions found in the dead link https://web.archive.org/web/20110123084336/http://developer.amd.com/gpu/AMDAPPSDK/downloads/Pages/default.aspx

@ctrueden wrote:

Ah yeah, that's a good idea!

What would be really slick would be something like: {% include deadlink url='...' %} and then it links to web.archive.org, but with a little "broken link" icon after the link, which has a tooltip explaining that this is a dead link that might have a newer live replacement, and the reader is encouraged to edit the page to improve it.

[ctrueden]

I added this to the post-production milestone because it would make sense to implement this in conjuction with finishing #50.