Init support for entity-relationship diagram creation (Ruby ERD)

[Splines]

Entity-relationship diagrams (ERD) have long been a very powerful tool for gaining insights and an overview of how the tables in a database are structured and associated to each other.

Rails ERD generates such diagrams based on runtime-informations using reflection (instead of static analysis). We have already specified this dependency in our Gemfile, but not used it so far.

Tip

In order to generate the PDF files locally, see the new Wiki entry here.

For reviewers

• Merge the voucher PR Vouchers for user promotion - Part 2: Redemption #671 into this branch locally to be able to create the voucher-related ERDs. Otherwise you will encounter errors like raise NameError.new(message, name).
• Please make sure to rebuild at least the mampf dev docker container, as graphviz was added as dependency. This is a visualization library that Rails ERD uses to generate the actual PDF out of .dot files. We need to make sure it is available in the dev docker container.
• I've created some diagrams that I think could be useful as a good starting point. We can easily add more later on if needed. But feel free to add more diagrams directly in this pull request ;)

Sample output

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.45%. Comparing base (76e3329) to head (8cb3c43).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #697   +/-   ##
=======================================
  Coverage   54.45%   54.45%           
=======================================
  Files         160      160           
  Lines        6752     6752           
=======================================
  Hits         3677     3677           
  Misses       3075     3075           

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[fosterfarrell9]

Very nice! LGTM.