Refactor Lambdas to use queue prefix and reuse logic

[AntoineAugusti]

#188 introduced a new variable to hold the SQS queue prefix. We should use this in SES and SNS Lambdas.

On top of this, it would be good to refactor code and put methods in a common lib because we often need to get a queue and push Celery envelopes into it in all these Lambdas.

We could have a small Python file holding this logic that would be included in all 3 Lambdas.

[AntoineAugusti]

Ping @jimleroyer, as requested #188 (comment)