Optimize assets for size

[toofishes]

I broke this into individual commits in case any of them are seen as undesirable.

For full disclosure- I will next try and implement OpenEVSE/ESP8266_WiFi_v2.x#160 in this repository, but wanted to ensure there was no concerns about asset bloat. By reducing the current assets in size by 8KB, I figured adding additional assets would be more acceptable.

Here are the two meaningful commits and descriptions:

Run optipng on favicon-32x32 to shrink size

The 16x16 favicon was already fully optimized, but the 32x32 image was
not. This saves 142 bytes, which isn't much, but very little reason not
to do so.

Run svgo SVG optimizer on wifi images

These XML files contain a bunch of inkscape metadata that is not needed
to render the image. Use the svgo utility (https://github.com/svg/svgo)
with default options to remove all unnecessary bits from the files,
saving roughly 8 KB across the five images.

Obviously it is possible I missed something, but I did a visual
verification that nothing changed in the rendering of these images. I
cannot test in every possible browser, however.
You can do the "rich diff" option in Github on this PR to view the old and new images side by side.

[toofishes]

I noticed in another PR you said to rebase to v3_gui, so I did the same here. Let me know if you need it changed to master or anywhere else and I can do that!

[glynhudson]

Thanks 👍