Refresh app icon's design

[gnprice]

We have an updated design for the app's icon, with more-saturated colors in the background gradient:

This issue is for integrating that new icon into the app.

Implementation

The next step needed is to produce SVG files describing the new icon — both the production version shown above (which we'll use when this app rolls out to everyone), and a "beta" variant which we'll use for the remainder of the beta period.

Then we can run the "beta" version through our tools/generate-logos script, and it should do the job. Later we'll run the production version through tools/generate as part of preparing the rollout (#715).

Details in chat thread.

Related issues

• The beta version of the icon is expected to also fix this other issue: Nicer "beta" app icon #415

• The legacy app has two long-open issues for supporting some fancy features of app icons in modern Android:
  Provide an "adaptive icon" for Android launchers zulip-mobile#4211
  Add monochrome icon for Android launchers zulip-mobile#5881

  I believe those will be straightforward to resolve using the new SVG files; details in chat. We might not do so at the same time as fixing this issue, though.