Bug 21025: Add GetPreparedLetter to Koha::Patron::Discharge

Bug 21025: Add GetPreparedLetter to Koha::Patron::Discharge

Test plan (shamelessly ripped from Bug20953):

1) Set system preference 'useDischarge' to 'Allow'
2) Choose a patron without checkouts nor fines nor restrictions
3) Log at OPAC and go to patron page /cgi-bin/koha/opac-user.pl
4) Click on 'ask for a discharge' tab
   => You see /cgi-bin/koha/opac-discharge.pl
      with text 'What is a discharge? ...'
5) Click on 'Ask for a discharge' link
   => You see /cgi-bin/koha/opac-discharge.pl?op=request
      with text 'Your discharge request has been sent ...'
6) In a new browser tab/page, go to intranet on /cgi-bin/koha/members/discharges.pl
   => You see one discharge requets for the patron
   => Click on 'allow' on patron discharge request
7) Come back to OPAC and refresh /cgi-bin/koha/opac-discharge.pl
  => You see link 'Get your discharge'
8) Click on the link to get your generated PDF
  => An error message should appear informing you to contact your website's admin
9) Apply this patch
10) Click again on the link to get your generated PDF
  => This time, a download prompt should appear

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 654abf9966bd340689b1944a7e5266cde7127321)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>