Enable support for user-managed custom domains on cloud hosted version

[lanedirt]

The self-hosted AliasVault allows for adding custom domains via the install.sh configuration script. We want to also give users of the cloud hosted version the ability to use their own custom domain with AliasVault which will be a feature part of the "AliasVault Premium" offering.

Todo:

• Add .env toggle to enable/disable user managed custom domains.
• Add client page where user can configure their custom domains.
• Add API controller methods for configuring custom domains.
• Make sure that the custom domain can only be used (email claims added) by the user that owns it.
• Make user-managed custom domains feature part of a new premium account offering that will only be available on the cloud-hosted version.