Bug 37039: Update discharge request with CSRF token

Bug 37039: Update discharge request with CSRF token

The OPAC discharge page used a link with a GET parameter, but the script
expects a POST request. This patch converts the link to a form with CSRF
token included.

To test, apply the patch and enable the useDischarge system preference
if necessary.

- Log in to the OPAC as a user with no checkouts or outstanding fees.
- Click the "Ask for discharge" tab in the sidebar of the user summary
  page.
- Click the "Ask for a discharge" button.
- You should be redirected to a page that says "Your discharge request
  has been sent."

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>