Add ability to disable registrations

[pixeldesu]

A little present to the people that decide to host Retrospring themselves.

This is handled via a new configuration option in justask.yml:

features:
  registration:
    enabled: true

Removes all mentions of signing up from the interface (I hope) and redirects away from the sign up/user creation routes if they are tried to be accessed.

---

Important: This doesn't come with an invite system, so to create users you either temporarily enable registrations again, or create them manually over RailsAdmin.

[pixeldesu]

Rubocop smh:

• Complains about a not defined new in before action condition
• Defining the above would literally just be a new action with super
• Rubocop strips that because that's redundant

---

@nilsding I'm out of clues why the redirect doesn't seem to apply to the create action POST request. Maybe you have an idea.

[nilsding]

One thing I noticed: the links in the sign in page still refer to "Sign up" as well

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.91%. Comparing base (2001921) to head (49c4371).

Files	Patch %	Lines
app/controllers/user/registrations_controller.rb	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1824      +/-   ##
==========================================
+ Coverage   95.89%   95.91%   +0.01%     
==========================================
  Files         179      180       +1     
  Lines        2803     2816      +13     
==========================================
+ Hits         2688     2701      +13     
  Misses        115      115              

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[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ **ActionView::Template::Error: undefined method registrations_enabled' for Retrospring::Config:Module (ActionView::Template::Er...** UserController#show` View Issue

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