fix(auto-prefixer): resolve perf impacts as reported by LightHouse

[ThomasBurleson]

• Order of flex properties should be improved to use display: flex last.
• missing standard property display: -webkit-flex;
• mistakenly using display: -moz-box;

See

• Explanation of old CSS flexbox + advice GoogleChrome/lighthouse#1710 (comment)
• Audit: Prefixing Flexbox is currently considered an error GoogleChrome/lighthouse#2022 (comment)

[ethanroday]

@ThomasBurleson, removal of --webkit-box in #283 means that flex layouts will no longer work in UC Browser, which has ~10% share WW.
Was there a functional reason for this change, or was it just to cut down on CSS?

[jelbourn]

We aim to mostly support "modern" browsers- ultimately we have to draw the line somewhere. In this case we decided that supporting the final flex spec is necessary to be considered "modern".

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