Modify browser support check, move it outside angular

Given that stellar-lib throws exceptions on older browser upon evaluation, IMO we need to move the unsupported browser detection to a spot before we try to load stellar-lib.

This PR does that:

-  It moves the "unsupported browser" state to a separate html file: unsupported.html
- Moves the logic of checking browser support into the global function "isSupportedBrowser"
- Adds check for DataView, which is required by stellar-lib and its dependencies