initialize correspondence

[Andreass2]

Description

• Change Id's to Guid
• add database models needed to initialize Correspondence

Related Issue(s)

• Implementere databaselag for å opprette meldinger #43

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)
• All tests run green

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)