Fix new lecture creation

[Splines]

Closes #738. The problem was a wrong multiple: true statement introduced in the 2nd voucher PR #671. Beforehand this attribute wasn't set and thus defaulted to false (like it is supposed to be).

I've also added a new cypress test to test that admins can create new lectures. To do so in an empty database, I had to give admins the ability for this, even when they are not course editors. (In the future, we could think of simply giving admins rights to every endpoint.) I've also had to adjust another test that made use of the helper function where the multiple: true was used. In that case, the behavior was a bit broken too, since we allowed users to select multiple teachers although just one is accepted by the backend.

Out of scope:

• In the test, I had to select an option from a TomSelect dropdown. I've added a respective cypress command that we can reuse from now on.
• I also added a missing right to docker buildx.
• Furthermore, we set the experimentalRunAllSpecs flag to true such that we can through all cypress tests at once locally.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 54.26%. Comparing base (23aae01) to head (8df92a3).
Report is 1 commits behind head on release/1.15.2-first.

Files with missing lines	Patch %	Lines
app/abilities/lecture_ability.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           release/1.15.2-first     #741   +/-   ##
=====================================================
  Coverage                 54.26%   54.26%           
=====================================================
  Files                       170      170           
  Lines                      7356     7356           
=====================================================
  Hits                       3992     3992           
  Misses                     3364     3364           

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[fosterfarrell9]

LGTM.