add logo in SVG format

[sisp]

I've created a logo in SVG format by manually re-drawing the PNG one. It isn't pixel-identical:

• The edges weren't perfectly sharp in the PNG version, so I had to guess the exact coordinates of the path points. The brackets are identical, the right one is the flipped version of the left one. The triangle in the middle is horizontally symmetric, the gaps are exactly parallel to the triangle's line on the left and have identical offsets to their neighbors.
• The triangle in the middle wasn't perfectly centered vertically and possibly horizontally, but I assumed it should be, so I centered it.
• I tried to guess the parameters of the gradient by extracting its start and stop color and its angle, but it's not 100% identical to the PNG version.
• I cropped the canvas so the shape touches the boundaries of the viewport.

I've also updated the PNG version and the favicon, so they are all based on the SVG image.

What do you think?

Related to #433.

[codecov]

Codecov Report

Merging #435 (7bc1ba7) into master (ddfc4de) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #435   +/-   ##
=======================================
  Coverage   90.90%   90.90%           
=======================================
  Files         179      179           
  Lines        6389     6389           
  Branches     1157     1168   +11     
=======================================
  Hits         5808     5808           
  Misses        544      544           
  Partials       37       37           

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[xaviergonz]

looks awesome!