[native] Highlighting edited message

Summary: We set background of the edited message to 25% of the thread color.

Test Plan:
Run the app on iOS, and check if the edited message is highlighted.

Screenshots:

Reviewers: michal, inka, ginsu

Reviewed By: inka

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7447