Add devices-16.svg, devices-24.svg

[stvehayes]

Closes https://github.com/github/primer/issues/1491

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Latest commit: b77a8ca57a8cf70410d1c93bbfcdb2454bdc0558

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[stvehayes]

The note that I had from the original meeting was "devices" over "sessions" because it matches the other Octicon naming. I barely remember talking about it, one of the keywords is "sessions" though.

Updated the JS file with "DevicesIcon". If anyone feels strongly about naming it Sessions instead of Devices, I can revise it. @gavinmn @tallys