Allow dynamic instance method calls on FactoryBot objects in Cypress

[Splines]

Cypress API changes

A first API for this was proposed in #671. This is a redesign of that API to make things more scalable and easier to use from within Cypress tests. With the changes in this PR, we are able to write something like this in Cypress tests:

FactoryBot.create("lecture", { teacher_id: this.teacher.id }).as("lecture");

cy.then(() => {
  this.lecture.call.long_title().then((res) => {
    cy.log(res);
  });
});

Call .call on any instance you've created with FactoryBot. On the resulting object, you can call any method, e.g. here long_title(). This will translate to a method call on the respective instance in the backend.
_{You can even pass (multiple) arguments to this method, but of course this only works for primitive data types such as strings and integers and not when the function excepts a user object or other complex Rails objects.}

For all supported method calls, see the respective tests under cypress/e2e/meta/factory_bot_spec.cy.js.

My plan is to merge this PR into dev, then remove the first proposal API from #671, then merge dev into #671 and use the new API there for the tests. This also makes PR #671 smaller.

Off-topic

• In Cypress, we can now create multiple users of the same role, e.g. multiple generic users. This is achieved by adding a random hash to their email such that there is no duplicate key.
• In the Cypress JavaScript files, we don't store the default Cypress password anymore. Instead, it is returned by the backend via the controllers/cypress/user_creator_controller.rb.

---

For reviewers

• It's not worth to browse through my commits here as it's only in the last ones that everything works. Before that, I experimented a lot.
• Convention in JS: methods that should be considered private start with a #.
• There is a lot of "JavaScript magic" going on here. In order to understand the mechanism with a simpler example, read through my question here. The answers to this questions were the reason I introduced the .call syntax instead of allowing dynamic method calls directly on the resulting object. The function defined in the StackOverflow question is very similar to the final one in this PR, namely #allowDynamicMethods() of the factorybot.js. Setting the .call property is done in #defineCallProperty().
• In the code here, the complexity increases because of how Cypress handles things in the .as() and .then() methods with its own wrapping mechanisms. This is handled in #createProxy().
• To see how multiple arguments could be handled, see this commit. You might want to check out one commit before that to test it locally, i.e. git checkout f20d8120dc.

Out of scope

• This PR also refactors the create() method of the Rails FactoriesController according to some changes already done in Vouchers for user promotion - Part 2: Redemption #671, e.g. we introduce the method validate_factory_name.
• Do we also want to keep the transform_hash method? Could it still be useful? If yes, could you please add a docstring explaining what it does, e.g. input/output as I'm not too sure 😅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.45%. Comparing base (49cf16a) to head (6ed2190).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #696   +/-   ##
=======================================
  Coverage   54.45%   54.45%           
=======================================
  Files         160      160           
  Lines        6751     6752    +1     
=======================================
+ Hits         3676     3677    +1     
  Misses       3075     3075           

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[Splines]

@fosterfarrell9 Thanks for your review, I've incorporated all your suggestions. Please take note of the "off-topic" section I've introduced in the PR comment.

[fosterfarrell9]

LGTM!