[FIX JENKINS-38175] Fix various ManagementLink related bugs

- In the context menu, the 'post' flag was set iff 'requiresConfirmation'
  was set, even though they're independent (e.g. Prepare for shutdown
  requires the former but not the latter)
- /manage screen: The icon (t:summary) does not support POST or
  confirmation links, but was set to not link only if no confirmation
  was required (i.e. POST links did not POST when the icon was clicked
  -- now the icon is not clickable as a workaround)
- /manage screen: All links requiring confirmation did POST, which masked
  the fact that the 'Reload from disk' link wasn't set up to require POST
  (it was only broken in the context menu). Now, confirmation and POST are
  separate flags, and 'Reload from disk' link now requests POST.