Add System Info to the reference application UI

Story

As a system manager, I would like to know the version of the OpenMRS platform and modules that I am running, so that I have basic information, and to help when debugging errors.

Acceptance Criteria

1. As a privileged user, when I click on the "System Administration" link on the homepage I should see a "System Info" link.
2. As a privileged user, when I click on the "System Info" link I should see a page that shows:
   • OpenMRS Core version
   • Names and versions of all running modules
   • Operating System version
   • Total (and free?) memory
   • a "Copy to clipboard" button
3. When I click the "Copy to clipboard" button on the System Info page, the information shown on this page should be copied to the clipboard in a format suitable to be pasted into an OpenMRS Talk message. (E.g. markdown-formatted text.)

Dev Notes

The UI for this should be added to the adminui module. Required API methods should be added at a lower level if possible, e.g. getting a list of modules and versions via REST should be added in the webservices.rest module.

The dev who picks up this ticket should briefly research what type of information can be easily collected and displayed on this screen, and suggest revisions to what's written in the ticket now.