Define the browser support matrix

[stasm]

What is the intended browser support matrix for https://send.firefox.com?

I found the note in the FAQ, Why is my browser not supported? which pointed me towards the WebCrypto API. Unfortunately MDN's compatibility table is missing a lot of data.

I'm asking because in projectfluent/fluent.js#133 we're considering dropping the support for old versions of Chrome and Firefox as well as the support for IE11. Fluent is used the localize the Firefox Send website. Dropping the support would mean that the fluent/compat builds might contain async functions and function* generators, which cause syntax errors in browsers which don't support them.

Here's the list of browser versions we're currently considering supporting in Fluent's compat builds:

Firefox >= 52,
Chrome >= 55,
Edge >= 15,
Safari >= 10.1,
iOS >= 10.3