More accurate browser warnings

[chillu]

Overview

Followup from https://github.com/silverstripe/silverstripe-framework/issues/6719

Acceptance Criteria

• As a user with a potentially incompatible browser, I can get informed about better choices
• I can discard the warning, and the choice is remembered on my device
• If my device runs an unknown or explicitly unsupported browser, I can still choose to continue
• I don't get blocked from doing my work, even though some functionality might be broken

Notes

• Browser support is a bit of a continuum. Defining it as "browsers we list as supported and actively test on" is a fairly narrow view. For example, latest Opera is using WebKit so is quite likely to work, but older Opera with their own rendering engine will almost certainly break.
• Based on features, we're dependant on Flexbox and SVG support
• We're still testing Behat with Firefox 31, so technically can't mark this as unsupported
• We don't list Safari as a supported browser, but in fact we encourage mobile viewports in the CMS now (at least on tablet resolution). So realistically that means supporting at least Mobile Safari, and likely modern Android
• There's some frontent build tools where we can encode browser support, ideally we can sync that up with what's implemented here.

Browser Checks

Implement as a whitelist - meaning other user agents will get the warning as well (e.g. Opera Mobile)

• = IE v11 (mostly Flexbox compat)

• = FF v28 (start of Flexbox support)

• = Opera v40 (v21 is start of Flexbox support, around the time they switched to WebKit/Blink. v40 was released a year ago)

• = Safari v9 (only support newer versions)

• = Android Browser v4.4 (start of flexbox support, we should probably do some sanity checks there though)