[web] Use new style prop passed from getMessagePreview

Summary:
This diff results in only one visual difference: when the most recent message in a read thread is a robotext message, it is displayed with the darker "secondary" color. This brings web behavior to match native, but I'm also open to the alternative (making native behave like web).

Will annotate CSS changes inline, as they're somewhat complicated.

Depends on D6357

Test Plan:

before	after

(identical except "ashoat added commbot" near the bottom)

Reviewers: inka, ginsu, atul, tomek

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D6358