backoffice: limit xls cell content to 32767 characters (#7056)

Total number of characters that a cell can contain: 32,767 characters

source: https://support.office.com/en-nz/article/Excel-specifications-and-limits-16c69c74-3d6a-4aaf-ba35-e6eb276e8eaa

wcs/backoffice/root.py

@@ -988,6 +988,10 @@ class FormPage(Directory):
 
                 for i, filled in enumerate(items):
                     for j, elem in enumerate(self.formpage.csv_tuple(fields, filled)):
+                        if elem and len(elem) > 32767:
+                            # xls cells have a limit of 32767 characters, cut
+                            # it down.
+                            elem = elem[:32760] + ' [...]'
                         ws.write(i+1, j, elem)
 
                 self.output = cStringIO.StringIO()