Keep track of the database schema version

The singleton table SchemaVersion contains the current version
of the Hydra database schema.  This can be used to upgrade the
schema on the fly.

Also reran the DBIx::Class schema loader.

src/sql/hydra.sql

@@ -1,3 +1,11 @@
+-- Singleton table to keep track of the schema version.
+create table SchemaVersion (
+    version       integer not null
+);
+
+insert into SchemaVersion (version) values (1);
+
+
 create table Users (
     userName      text primary key not null,
     fullName      text,