Require IE11+ for platform

[reefdog]

We already decided to end support for IE8 and older (see conversation starting here). Shedding IE9-10 and requiring IE11+ would free us up even more, letting us shed some shivs and polyfills and start using flexbox.

IE9-10 were 0.7% of our traffic over the past year. (IE8 actually gets more than each one individually.) Microsoft no longer supports them. Adding them to our unsupported list doubles our unsupported IE traffic from 0.7% to 1.4%. I think this is all reasonable.

[reefdog]

I should note that this is only for the platform, not the embeds, where we could be a little more generous. Although of course any iframed embeds would de facto adopt the platform's requirements.

[knowtheory]

always helpful to refer back to https://analytics.usa.gov/ too.

Although frankly at this point i'm fine moving forward.

[reefdog]

Yup, that's a useful proxy for our embeds particularly since it represents "the wider web" better than our platform numbers. They've got IE10- at ~2.1%.