replace dynamodb for contact messages

[exceptionalnull]

After finishing the build it seems that DynamoDB is both overkill and a bit limited for this use case. Replacing it with a custom S3 backed service will be a bit more work but will allow for more features and probably be cheaper.

[exceptionalnull]

Making the DynamoDB pagination work like the eventual pagination probably will was too much effort for what would be a costly solution. So it uses the lastEvaluatedKey method for now. This just means the method signatures for retrieving the list of messages will need slight alterations when switching to the new backend.