Update browserslist and 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 browser list db with the most up to date 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 decreased (-83.7%) to 0.0% when pulling fa0e8bf8b32c0c305b6ec98cefdea29c50997e70 on msal-react-browserslist into ee64805 on dev.