Using an if block when {!event-filtering-additional-feature.md!}
is used implied that we plan to have this in all webhook doc .md
files.
But, it actually makes sense to only use this macro when the
webhook integration actually supports event filtering.
markdown-include is GPL licensed.
Also, rewrite it as a block processor, so that it works correctly
inside indented blocks.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
