Allow read only stores

@staugaard @osheroff @gabetax 

Currently we configure an EU or US S3 stores for emails depending on the pod location. It works fine for looking up emails in corresponding stores except when we are moving accounts. We need to configure a readonly store, in addition to the primary store, so that the emails can be looked in the backup store when an account is being moved. For example, if we are moving an account from US to EU, until we have moved all the emails to EU store, we should be able to look for emails in the US store. If we don't have a backup store, emails won't be accessible till we have moved all of them. 

The readonly option here is important as we don't want to start copying all the emails (and their attachments) for all accounts to both US and EU stores.



I was looking for a better name here. Like `read_write_stores`. I will change it.