Conditionally run post-attachment migrations

Introduce placeholder migrations for Backbone models so they never implicitly
run migrations whenever they are `fetch`ed. We prefer to run our migrations
explicitly upon app startup and then let Backbone models be (slightly) dumb(er)
models, without inadvertently triggering migrations.

js/modules/migrations/get_placeholder_migrations.js

@@ -0,0 +1,23 @@
+const Migrations0DatabaseWithAttachmentData =
+  require('./migrations_0_database_with_attachment_data');
+const Migrations1DatabaseWithoutAttachmentData =
+  require('./migrations_1_database_without_attachment_data');
+
+
+exports.getPlaceholderMigrations = () => {
+  const last0MigrationVersion =
+    Migrations0DatabaseWithAttachmentData.getLatestVersion();
+  const last1MigrationVersion =
+    Migrations1DatabaseWithoutAttachmentData.getLatestVersion();
+
+  const lastMigrationVersion = last1MigrationVersion || last0MigrationVersion;
+
+  return [{
+    version: lastMigrationVersion,
+    migrate() {
+      throw new Error('Unexpected invocation of placeholder migration!' +
+        '\n\nMigrations must explicitly be run upon application startup instead' +
+        ' of implicitly via Backbone IndexedDB adapter at any time.');
+    },
+  }];
+};