add minimal IE11 support

Fully supporting IE11 is challenging due to it lacking support for the Javascript and CSS features we use. This PR adds enough polyfilling and tweaks to get the application functional but not terribly pretty. A notable loss is the box shadow on the asset entry table.

For the moment, I've put in the banner from browser-update.org in lieu of better wording. I feel we should find some words to give to IE11 users to prepare them that the experience may be a little sub-par but I don't know what they should be. If this PR is merged, we should open a design task issue to find what words we use.

Closes #99 (closed)