Reworked UnprotectedRootAction display to be in configure page for better discoverability and possibility of support from reverse-proxy-auth-plugin.

Fixed typo in sample command line args.

Implementation according to Jesse Glick's comments on the issue.

[FIXED JENKINS-15889] Saving the update center list after the metadata has been fetched results in the metadata being persisted twice

(copied from the mailer plugin)

... instead of creating a copy of each GlobalConfiguration variant (like GlobalSecurityConfigurationPart) for the following reasons:

- This allows future UI to render tabs and other means to divide configurations into even smaller granularity, which is something that has been discussed before.

- Eliminates code duplication between GlobalConfiguration look-alikes.

- All our other icons do not have bounding boxes, so let's follow that
  scheme as well.
- Changed the name to "Configure Global Security" to make it clear that
  this is a part of the configuration.

also handles usernames that are emails