Store preferences per user

[OliverAbdulrahim]

Store the following settings by user:

• Preferred sync interval
• Email templates (release, receipt, update)

[OliverAbdulrahim]

As part of this enhancement, explore the scope and usefulness of implementing the following:

• Portfolio IDs as configuration rather than environment variables
• Potentially, access token as configuration rather than environment variable
• Error message when access to these is required but unspecified