Bug 10951 - Adding NoLoginInstructions to system preferences

Bug 10951 - Adding NoLoginInstructions to system preferences

     On a failed login, the default message is harcorded into opac-auth.tt.

     It would be preferable to allow for a preference to override that message (for example: ...Please bring an ID to t
     The changes modify
         -opac-auth.tt to allow for custom value
         -admin/preferences/opac.pref to add it to the preferences with a description
         -C4/Auth.pm for the loading of the preference
         -sysprefs.sql
         -updatedatabase.pl

     TESTING
         1) in OPAC, logged out, try login in by entering no or wrong credentials.  Acknowledge the "Don't have a p
         2) Apply the patch
         3) Regression Test: Redo step 1.  Same (default) message should appear.
         4) Log in to intranet,
             - select NoLoginInstructions in system preferences.
             - Enter new (xml) message.  Possible:
             <h5>Welcome to Koha, please bring your passport to the front office</h5>
            - and save
         5) refresh the OPAC, try login again with invalid credentials.  The new message should appear.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>