Fix CSRF exception when updating theme

What does this MR do?

Properly trigger rails-ujs submit when changing themes.

Without this, clicking on a theme resulted in a CSRF error because the form wasn't submitted remotely and Rails doesn't insert the hidden field to the form if remote: true is set.

This wasn't caught by our specs because CSRF protection is disabled in test mode and the controller works fine with a regular submit and the result to the user looks the same so our assertions still passed.

This MR also enables CSRF protection for feature tests so that we catch these problems in the future.

Related to #254826 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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