Add canonical URL for Augur's annotations schema

Matches the new schema id (to be) in Augur, see
<nextstrain/augur#1281>.

Resolves <#704>.

Author: tsibley

Diff for: src/app.js

@@ -418,6 +418,7 @@ app.route("/cli")
  * to programmatic clients, though we're not this fancy yet.
  */
 const schemaRoutes = [
+  ["/schemas/augur/annotations",       "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-annotations.json"],
   ["/schemas/augur/frequencies",       "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-frequencies.json"],
   ["/schemas/auspice/config/v2",       "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-auspice-config-v2.json"],
   ["/schemas/dataset/v1/meta",         "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-export-v1-meta.json"],