Calendar Popup: Fix prev/next month for shorter months

Jump to correct month if the prev/next month has fewer days (e.g. 28/29/30)
than the currently selected day (e.g. 29/30/31)

Based on a fix by Ludwig Reiter <ludwig.reiter@intevation.de>

diff -r e328ffaa54ce roundup/cgi/templating.py
--- a/roundup/cgi/templating.py	Thu Sep 19 16:44:33 2019 +0200
+++ b/roundup/cgi/templating.py	Thu Nov 14 13:00:49 2019 +0100
@@ -3121,13 +3121,32 @@
             display = current_date
         day       = display.day
 
+        def prev_month_interval(display):
+            # jump to correct month if the prev month has fewer days
+            # (e.g. 28/29/30) than the current display.day
+            return date.Interval("-%dd" % (display.day + 1))
+
+        def next_month_interval(display):
+            # jump to correct month if the next month has fewer days
+            # (e.g. 28/29/30) than the current display.day
+            year = display.year
+            month = display.month
+            if display.day > 28:
+                if month == 12:
+                    year += 1
+                    month = 1
+                else:
+                    month += 1
+            days = calendar.monthrange(year, month)[1]
+            return date.Interval("+%dd" % days)
+
         # for navigation
         try:
-            date_prev_month = display + date.Interval("-1m")
+            date_prev_month = display + prev_month_interval(display)
         except ValueError:
             date_prev_month = None
         try:
-            date_next_month = display + date.Interval("+1m")
+            date_next_month = display + next_month_interval(display)
         except ValueError:
             date_next_month = None
         try: