process_message_event: Deduplicate `is_notifiable` logic.
process_message_event: Deduplicate `is_notifiable` logic.

We use the `UserMessageNotificationsData` methods to check if the
message is notifiable, and use a cleaner early continue pattern.

eeshangarg/zulip,zulip/zulip,punchagan/zulip,kou/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,andersk/zulip,rht/zulip,rht/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,kou/zulip,andersk/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,andersk/zulip