We want to call `set_message_booleans` as soon as we
get data from the server, to avoid confusion about whether
`flags` is the authoritative field.
This commit has callers to `add_message_metadata` call
`set_message_booleans`.
This also sets us up to **not** call `set_message_booleans`
in the local echo codepath, where we can just have the
markdown processor set booleans natively.