[JENKINS:SECURITY-982] CSRF vulnerability and missing permission checks in Confluence Publisher Plugin

Confluence Publisher Plugin did not perform permission checks on a method implementing form validation.
This allowed users with Overall/Read access to Jenkins to submit login requests to Confluence using attacker-specified credentials.

Additionally, this form validation method did not require POST requests, resulting in a CSRF vulnerability.

This form validation method now require POST requests and Overall/Administer permissions.