Fixing IE8, 9, 10 support

[cmc333333]

eRegs currently breaks in IE8, 9, and 10 (it's fine in IE11). With some preliminary testing, a significant chunk of this problem rests in our inclusion of prosemirror (which is included for notice and comment). There are a few things we should/can do to fix this:

• We currently only run integration tests on Chrome and IE11. Expand this to the browsers we actually want to support
• ProseMirror is out of date. Hypothetically updating it would fix an issue with IE10. This may not be relevant if we implement the next point.
• Perhaps more importantly, ProseMirror isn't required for any active site right now. It'd make sense to split that component out. The Python's being split in Split comment-related functionality into separate Python module eregs/regulations-site#491, but the JS is always included.
• We probably want to include the babel-polyfill to account for ES6 features not present in earlier browsers (though we may not be using those features yet).

[cmc333333]

Collectively, IE8, 9, and 10 make up less than 1 percent of the DOJ's audience. While https://github.com/eregs/regulations-site/tree/485-split-nc-js fixes IE10, let's hold off until we have integration tests for these browsers.