users: change default user level to member in database.

Fixup for 9c34d5cc3. Default to member level in the database because
setting it in `customize_new_user` clobbered user levels set by factory
bot in tests, which broke the test suite.

db/migrate/20190923071044_change_default_level_on_users.rb

@@ -0,0 +1,5 @@
+class ChangeDefaultLevelOnUsers < ActiveRecord::Migration[6.0]
+  def change
+    change_column_default(:users, :level, from: 0, to: 20)
+  end
+end