R1:3c65601285e9

Implement "@{config:...}" as a real Remarkup rule

Summary:
See <https://discourse.phabricator-community.org/t/firehose-webhook-not-working-with-self-hosted-requestbin-instance/2240/>. I want to make it easier to link to configuration from system text.

We currently use this weird hack with `{{...}}` that only works within Config itself. Instead, use `@{config:...}`, which is already used by Diviner for `@{class:...}`, etc., so it shouldn't conflict with anything.

Test Plan: Viewed config options, clicked…