Update RuboCop & fix offenses by new lints

[Splines]

We update RuboCop and fix problems caused by the new lints that were introduced.

[Splines]

@fosterfarrell9 Could you maybe look into the Style/SafeNavigationChainLength? I've adjusted the maximum value to 3 (default was 2) and we still have two offenses. We might want to reset the max value to 2, however I'm not too sure yet how to tackle the respective offenses. If we don't think this lint is useful, we could also disable it entirely.

To tackle the offenses, we'd have to know exactly which attributes are guaranteed to be non-null.

[fosterfarrell9]

I do not think this lint is useful. Rewriting this would make it unnecessarily bloated. Also, for the second one, we would have to dig into thredded. I suggest raising the maximum value to 4.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (dev@7dd3a33). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
app/helpers/submissions_helper.rb	0.00%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             dev     #714   +/-   ##
======================================
  Coverage       ?   54.30%           
======================================
  Files          ?      170           
  Lines          ?     7348           
  Branches       ?        0           
======================================
  Hits           ?     3990           
  Misses         ?     3358           
  Partials       ?        0           

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