Allow blank value for SODARUserAdditionalEmail.secret

[mikkonie]

Currently we enforce this as non-unique and non-null. However, when syncing verified alt emails for target sites, the value is useless as the verification has already been performed. Hence, it makes no sense to sync the secret.

However, we must have a value, so as a workaround (see #1476) we create a dummy secret on a target site.

The model should be changed in a way that empty secrets are allowed. This requires a migration, so I'll do it in v1.1 to avoid messing up the squashed migrations.