[SECURITY-2786] Fixes for CSRF vulnerability and missing permission checks

[stuartrowe]

Description

See JENKINS-71912 / SECURITY-2786.

Added permission checks to methods implementing form validation and additionally annotated the methods to require POST requests.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description.
• Appropriate unit or acceptance tests or explanation to why this change has no tests.
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description.
• Reviewed the code.
• Verified that the appropriate tests have been written or valid explanation given.
• If applicable, tested by installing this plugin on the Jenkins instance.

[stuartrowe]

@nrayapati please review when you're able.