Fixes #1837 by no longer depending on the undocumented f:validateButton feature. I recommend squash-merging due to unclean PR commit history.

As a bonus, made the form field ID a little more unique, in case there'd be some other plugin with a secretToken field.

Testing done

Clicked the buttons in a freestyle config, see #1837. Saving still works, new value is effective on page load.

With CSP plugin installed, no new CSP findings in Manage Jenkins » Content Security Policy Report.

Screenshots

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After

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