Set up review apps for merge requests

We need to be able to test how it actually runs on Serverless before deploying to production, because apparently even running a docker image cannot fully simulate the environment.

The following discussion from !4 (merged) should be addressed:

• @godfat started a discussion: (+14 comments)

  @grzesiek This is giving me this error:

  {"error":"OpenSSL::SSL::SSLError","message":"SSL_connect SYSCALL returned=5 errno=0 state=SSLv3/TLS write client hello"}

  from: https://triage-customers.triage-serverless-12690061.triage.serverless.grzegorz.co/

  Any idea? Do we have certificates set up in serverless? Should I pin application certificates?