[JENKINS-25572] fix condition where admin can't run tests

If a project is set to "trigger only", and an admin adds the test/retest phrase in a comment, nothing would happen. The logs would display "Trigger only phrase but we are not triggered". This is because the `triggered` variable was set to true on every other instance of manually triggering a build, *except* when an admin made the comment.
[JENKINS-25574] accept whitelisted users existing PR's

if a user has a PR, *then* gets whitelisted, the existing PR's are still unaccepted.
[JENKINS-25603] avoid stale configuration

a bunch of `private final` values had to be replaced with private
methods, this is because the configuration could change after a PR is
made, but without this change each PR would still be using the
configuration as it existed when it was created.