webview updates: Factor out helper `equalFlagsExcludingRead`.
webview updates: Factor out helper `equalFlagsExcludingRead`.

This helps make the logic a bit easier to read; it'll also help us
adjust the plumbing of how the `flags` data gets passed around here.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile