Update color palette to be more colorblind friendly

[lauralindzey]

I plan to add a 4th color to our symbology to represent "downloaded", and as part of that I want to review the symbology for whether it works people with various forms of colorblindness.

Our current symbology is:

• available: #1f78bc (blue)
• unsupported: #888888 (grey)
• unavailable: #fb9a99 (salmon)

Using the visualizer from davidmathlogic.com/colorblind:

Not terrible, but choosing a 4th color was hard, and I would like "unavailable" to be more perceptually different.

New proposal: Using colors from the Okabe + Ito palette

• downloaded: #009E73 (green)
• available: #0072B2 (blue)
• unsupported: #666666 (grey; not technically in this palette, but black is too dark)
• unavailable: #E69F00(orange)

Again, using the same visualizer

Closing this issue will require updating the script that generates the .qlr file.

[lauralindzey]

Now that the plugin is modifying the layer styling, it might make more sense to make this a configuration option, rather than choosing a fixed set of colors. I think this would be a straightforward change.