Deviation template tags

This commit includes new template tags for calculating values displayed in
tables in the deviation override update.

Fixes apluslms#896
Fixes apluslms#902

Author: skulonen

deviations/templatetags/__init__.py

@@ -0,0 +1,37 @@
+import datetime
+from typing import Optional
+
+from django import template
+
+from ..models import DeadlineRuleDeviation
+
+
+register = template.Library()
+
+
+@register.simple_tag
+def new_deviation_minutes(
+        deviation: DeadlineRuleDeviation,
+        minutes: Optional[int],
+        date: Optional[datetime.datetime]
+        ) -> int:
+    """
+    Get the extra minutes for a deadline deviation after being overridden.
+    """
+    if date:
+        return deviation.exercise.delta_in_minutes_from_closing_to_date(date)
+    return minutes
+
+
+@register.simple_tag
+def new_deviation_date(
+        deviation: DeadlineRuleDeviation,
+        minutes: Optional[int],
+        date: Optional[datetime.datetime]
+        ) -> datetime.datetime:
+    """
+    Get the new deadline for a deadline deviation after being overridden.
+    """
+    if date:
+        return date
+    return deviation.exercise.course_module.closing_time + datetime.timedelta(minutes=minutes)