move getTemplateOrg method; enable template org in docker tests (#602)

* move getTemplateOrg method; enable template org in docker tests

This moves the `getTemplateOrg` method to a neutral venue for the
convenience of `grist-static`, otherwise a lot of awkward dependencies
get pulled in needlessly in new parts of the app.

This also fixes docker tests using the template org.

app/server/lib/gristSettings.ts

@@ -0,0 +1,13 @@
+import {appSettings} from 'app/server/lib/AppSettings';
+
+export function getTemplateOrg() {
+  let org = appSettings.section('templates').flag('org').readString({
+    envVar: 'GRIST_TEMPLATE_ORG',
+  });
+  if (!org) { return null; }
+
+  if (process.env.GRIST_ID_PREFIX) {
+    org += `-${process.env.GRIST_ID_PREFIX}`;
+  }
+  return org;
+}