DEV: Review fixes (#10641)

See comments in https://review.discourse.org/t/dev-imap-log-to-database-10435/14337/6 for context.

db/migrate/20200910020909_make_imap_sync_log_cols_not_null.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class MakeImapSyncLogColsNotNull < ActiveRecord::Migration[6.0]
+  def change
+    change_column_null :imap_sync_logs, :message, false
+    change_column_null :imap_sync_logs, :level, false
+  end
+end