Drop IE Support

[carmacleod]

As discussed on the Jan 21 call, APG will drop support for Internet Explorer moving forward.

Prose

There are 2 instances of the string "Internet Explorer" in the Browser and Assistive Technology Support section that need to be addressed. I will provide a PR for the prose shortly.

Code

There are 6 code files that contain the string "IE" and/or "Internet Explorer".
I don't think there's any point in deleting the instances in the deleted sections archive.
Would like others to have a look at the remaining 5 files to see if anything should be cleaned up.

Issues

There are 21 open issues that mention "IE" or "Internet Explorer":
#8, #34, #78, #243, #468, #557, #736, #811, #919, #1045, #1117, #1132, #1134, #1136, #1164, #1196, #1206, #1230, #1237, #1261, #1282
I have not looked at any of these yet - if anyone wants to help pare this list down, please feel free. :)