Use symbolic ref "HEAD" instead of "master" in the GitHub URLs for ou…

…r schema redirects

This is robust to changes of default branch name, which is helpful if we
ever make a blanket switch from "master" → "main" (as we really should).

src/app.js

@@ -418,15 +418,15 @@ 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"],
-  ["/schemas/dataset/v1/tree",         "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-export-v1-tree.json"],
-  ["/schemas/dataset/v2",              "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-export-v2.json"],
-  ["/schemas/dataset/root-sequence",   "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-export-root-sequence.json"],
-  ["/schemas/dataset/tip-frequencies", "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-tip-frequencies.json"],
-  ["/schemas/dataset/measurements",    "https://raw.githubusercontent.com/nextstrain/augur/master/augur/data/schema-measurements.json"],
+  ["/schemas/augur/annotations",       "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-annotations.json"],
+  ["/schemas/augur/frequencies",       "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-frequencies.json"],
+  ["/schemas/auspice/config/v2",       "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-auspice-config-v2.json"],
+  ["/schemas/dataset/v1/meta",         "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-export-v1-meta.json"],
+  ["/schemas/dataset/v1/tree",         "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-export-v1-tree.json"],
+  ["/schemas/dataset/v2",              "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-export-v2.json"],
+  ["/schemas/dataset/root-sequence",   "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-export-root-sequence.json"],
+  ["/schemas/dataset/tip-frequencies", "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-tip-frequencies.json"],
+  ["/schemas/dataset/measurements",    "https://raw.githubusercontent.com/nextstrain/augur/HEAD/augur/data/schema-measurements.json"],
 ];
 
 for (const [schemaRoute, url] of schemaRoutes) {