Remove Modernizr, keep html5shiv (Webpack edition)

[jessetan]

Modernizr has been part of the theme for a long time, although it is not used. Since its presence has downsides (reflow on load, increased theme size), we want to remove it.
To keep backward compatibility with older IEs, I've kept html5shiv in this PR so there is less chance of breakage.

This is a rework of #756 after our move to Webpack and friends. Rebasing did not make much sense.
Fixes #724, #525. Related #414. Supersedes #624 and #756

[agjohnson]

Change looks good to me!

I know this has been a long standing issue and we've been slow to adopt the change, so I don't want to keep holding up the work here. Perhaps just some screengrabs from older IE to verify that the experience isn't completely broken without modernizer?

[jessetan]

Any particular IE versions you want to see? Microsoft no longer offers free screenshots using Browserstack, that requires a paid account.
They do offer VMs with IE8 and up.

[agjohnson]

I tried saucelabs, but they dropped IE8 support. IE9 looked as expected though. I tried a demo account on browserstack, but couldn't test IE8. I even tried ie8 through wine to no avail.

I guess I'm not sure how thorough we need to be here then, testing sounds annoying. IE8 only accounts for 0.1% of RTD traffic, which also isn't 100% for projects using this theme.

[jessetan]

Perhaps the best way to get feedback on this is to release a dev version of the theme and to wait for user responses or bug reports.

[agjohnson]

Sounds good.