Make CSP compatible and remove YUI

[timja]

Followed https://github.com/jenkinsci/credentials-plugin/blob/master/docs/consumer.adoc for cleaning up the credentials code

Followed https://www.jenkins.io/doc/developer/security/csp/ for CSP

YUI spreadsheet for yui adaption: https://docs.google.com/spreadsheets/d/1UjvtFmNmEdjMN5DUoFxJfBryA8q-E5_HwOzVKbVG9b0/edit?gid=0#gid=0

Cleaned up the UI while I was touching this code, looks like lots of dead cruft from a previous migration that wasn't cleaned up

Testing done

Manually clicked the button and the dialog still appears

Submitter checklist

• [ ] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ ] Ensure that the pull request title represents the desired changelog entry
• [ ] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

Thanks @timja! This fixed part of the problem, but there are still a few more CSP violations tracked in #322.