model: Track/memoize stream-color variants somewhere other than `/api/model`

[chrisbobbe]

Currently (well, after #381), the Subscription class has a colorSwatch method to return a memoized StreamColorSwatch. That code is in /api/model.

Probably there's a better, more UI-focused place to do those computations (adjusting lightness, opacity, etc.) and provide them to consumers: https://github.com/zulip/zulip-flutter/pull/381#discussion_r1396528348