App version management.

* DB migration adds app_versions table for managing known app versions.
* /ping reads app version from User-Agent header and responds with latest version if the app is OKO and its version is older than the latest one stored.
* Added endpoint /app-versions which
  * lists all known versions via GET
  * adds/updates a version via POST

#3
Fix #4

server/sql_schema/V3_app_versions.sql

@@ -0,0 +1,4 @@
+CREATE TABLE app_versions (
+  version text NOT NULL PRIMARY KEY,
+  address text NOT NULL
+);