Allow adjusting project icon grayscale and opacity for settled/snoozed threads

[stevenlw-porpoise]

Project icons help me distinguish threads at a glance, but the grayscale and dimming make custom logos harder to recognize when looking back at my snoozed or settled threads. Also, I think adding custom project icons is a fun feature but not being able to see them properly on the sidebar takes some of the joy out of it.

Could Settings → Appearance include separate sliders for project icon grayscale and opacity? For example, 35% grayscale (65% color retained) and 50% opacity. The current values (100% grayscale, 40% opacity) could remain the defaults.

Related to #6842, which is based on a similar concern of threads from different projects blending together on the sidebar but that proposes a different solution of including the project label in the sidebar. This proposed solution is to continue showing just the icon but to make it more distinctive.