Feature request: Show a warning when using an unsupported browser version

It would be useful to display a warning to users whose browsers are not supported. This warning should not be too invasive, but it should be clear. It is likely that CyberChef will not function correctly in unsupported browsers anyway, so this warning should be triggered early, perhaps in the [preloader](https://github.com/gchq/CyberChef/blob/master/src/web/html/index.html#L34) code.

Supported browsers are currently:
- Google Chrome 40+
- Mozilla Firefox 35+
- Microsoft Edge 14+

This can be programatically checked in the `.babelrc` file [here](https://github.com/gchq/CyberChef/blob/master/.babelrc).

Any browsers which maintain compatibility with these supported browsers should function correctly and shouldn't be issued a warning. This can usually be determined from the user agent string.

The warning should suggest that the user try one of the supported browsers instead.