Correct activity icon colors, resolves #831

[t-schroeder]

Make activity icons black before applying color filters for final color.

Looking at https://codepen.io/sosuke/pen/Pjoqqp (which I assume the CSS filter calculation is based on, same as in Moodle core up until 5.0 (see MDL-83725)) I saw this paragraph:

For this code to work well the starting color needs to be black. If your icon set isn't black you can prepend "brightness(0) saturate(100%)" to your filter property which will first turn the icon set to black.

In Moodle 4.4 (see MDL-78284) the core activity icons got replaced by new non-black ones. Therefore the CSS filters result in incorrect colors. For some colors this difference is actually noticeable, especially if you have third-party activities with icons with base color black next to core activities with icons with this non-black base color. I'm fixing this in Moodle core through MDL-84306 and it is necessary to apply that same fix here.

[t-schroeder]

This is how the color ended up for me with:

setting	value
activityiconcolorassessment	#5B40FF
activityiconcolorfidelity	One shot (1 iteration)

[abias]

Thank you, @t-schroeder , for this PR.
As I have just commented in #831 (comment), I would like to wait for final feedback by Moodle HQ if this issue will be solved upstream in Boost Core or not. We will keep this PR on hold until then.

[abias]

Hi @t-schroeder , I stumbled again over this PR and looked at https://moodle.atlassian.net/browse/MDL-84306 again. Unfortunately, HQ did not reply there. Would you want to ping Ferran from your side again there?