markdown: Extract contains_problematic_linkifier.
markdown: Extract contains_problematic_linkifier.

Note we now avoid linkifier checks for the case that a message
contains more obvious backend-only syntax such as attachments.

The next commit will eliminate the ugly early-return.

rht/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip