Fixes for when link is both a download and external

Since only one background image can be displayed with CSS, the
padding-left was still present when the file was a download but the
image wouldn't come through because it was also external
(rel="external") so the padding left is not always "0" on rel external
to make sure there is no dead space.

Also added the external icon example to the master list of icons.
Merge pull request #2202 from cfarquharson/v3.0

Icons: Fixes for when link is both a download and external