integrations: Move `markdown` class to element which has markdown text.
integrations: Move `markdown` class to element which has markdown text.

We move the markdown class to the element which directly contains
the markdown text. This avoids markdown properties being applied
to other elements unintentionally like `h1` elements.

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