Fix SVG download URL resolution for IE/Edge

[radhikalism]

Extracts and decodes SVG source code from the generated data URL before saving SVG files using MS IE/Edge.

Without this change, the downloaded SVG files contain invalid data (the literal encoded data URL) instead of the original SVG source code.

[alexcjohnson]

@arbscht nice detective work!

We need to figure out a way to test this - probably should go in download_test but will need a bit of creative mocking. I can take a look at how to do that.

[alexcjohnson]

This is a definite improvement - from garbage output to actual SVG - but there's still an issue: somewhere we’re losing the # in clippath urls, so traces and shapes can overrun the margins. Investigating where that got broken... if it looks nontrivial we can merge this (at least you get a usable output, even if it's not perfect) but ideally I'd like to get the whole thing fixed at once.

[alexcjohnson]

Ah looks like we just forgot to put the # back in here

[alexcjohnson]

I don't have write access here so made a new PR to plotly.js -> plotly#2068