feat: Implement a fallback class for not implemented message types

[roschaefer]

Undraft and point the target branch to master once #26 is merged. The current target branch is just a convenience to make the diff a little smaller.

I included a hotfix commit by @mattwr18 in this PR. Other than that, here's my commit message:

feat: avoid NameError on unimplemented mess. types

@mattwr18 here is a test for your hotfix commit in the threema gem
repo itself.

@thorsteneckel we have deployed Matts commit already. Instead of
removing the DeliveryClass, I decided to leave it in place to avoid
integration issues. Is there something we could do about the delivery
receipts?

How could DeliveryReceipt class a little more useful?

[codecov[bot]]

Codecov Report

Merging #28 (e003489) into master (ab6a373) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #28   +/-   ##
=======================================
  Coverage   36.62%   36.62%           
=======================================
  Files          13       13           
  Lines         344      344           
=======================================
  Hits          126      126           
  Misses        218      218           

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[thorsteneckel]

Hi @roschaefer - thanks for this. Can you elaborate Threema::Receive::NotImplementedFallback a bit more. Why is this needed? A use case would be nice.

[roschaefer]

Okee, so I'm not supposed to change this code. @thorsteneckel could you also have a look at #29 and #30? They should have the nice side-effect on fixing the build.