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

[JENKINS-16069] Remove wrapping of JSON with ( )
Thanks !

now it is done properly on Javascript side so the current server
implementation triggers javascript errors.

Result can be observed on DynamicComboBox within ui-samples-plugin

[FIXED JENKINS-15126] Remove ";" from closing tag

After all, that's the installation the user is most likely going to choose, and if there's only one installation.

Plus when there's only one installation, there's no drop-down to select the Maven installation in the config UI, so the user is asked to do a rather unnatural "open config page" > "just click save" workflow.