Add db model for TestMessage database
Add db model for TestMessage database

Summary:
Adds a model for the testmessage table and support for getting the
message

Depends on D206477
Depends on D206407

Reviewers: anupc, paulruan

Reviewed By: paulruan

Subscribers: changesbot, kylec

Differential Revision: https://tails.corp.dropbox.com/D206480

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