popovers: Do not show option to mark unread for unsubscribed streams.
popovers: Do not show option to mark unread for unsubscribed streams.

In the message actions popover menu, adds an additional check for
whether the mark as unread option should be displayed based on if
the message is a stream message and if the user is subscribed to
the message's stream.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip