Improve icons for the different chat domains and channels

[axpoems]

Some highlights (quoting @HenrikJannsen from his comment in PR #1502):

• Update icons with bisq2 green.
• The meetup has the same icon as events. Maybe we can find a replacement there. Same for support/gen support.
• The icon used at off topic looks more like trade and the trade icon looks more like markets. Not sure if there is a good fit for off-topic of the current icons...
• The Bisq and BTC icons are a bit small. Also markets. For the BTC icon the typical tilted version with a circle might be also better.
• I am also not so sure if the support icon is clearly enough recognizable. Maybe some other icon which is more typical for support would be better.
• If we use a header, we could also consider to use colored icons in larger size, similar as the markets in offerbook. I guess the design could benefit from a few light color drops (as in trade protocols).

[axpoems]

You can assign this to me.

[djing-chan]

I think that can be closed