Add .browserslistrc and update docs around browser support

[tnorling]

• Updates docs to explicitly define what browsers we support
• Moves IE 11 caveats/known-issues from wiki to new markdown doc
• Adds .browserslistrc defining which browsers we support (this helps prevent certain polyfills for older browsers from being included)
• Adds postinstall step to update the browserslist db with the most up to date browser information (open to feedback around where this should go, it should happen at minimum before releasing)

Closes #3396 by removing polyfill that was causing a conflict with script-src CSP.

This was tested and does not break ie11 compatibility as long as the ie11 polyfills are still included in any apps that need to support ie11 (demonstrated in the react-router sample and ie 11 sample).

[coveralls]

Coverage remained the same at 83.837% when pulling 46aca3f on browser-support into 1c83dec on dev.