Impemente util function and templatetags to get week dates.

Advances BiscuIT-App-Alsijil#8.

biscuit/apps/chronos/templatetags/week_helpers.py

+from datetime import date
+
+from django import template
+
+from .util import week_days
+
+
+register = template.Library()
+
+
+@register.filter
+def week_start(week: int) -> date:
+    return week_days(week)[0]
+
+
+@register.filter
+def week_end(week: int) -> date:
+    return week_days(week)[-1]

biscuit/apps/chronos/util.py

-from datetime import datetime
-from typing import Optional
+from datetime import date, datetime, timedelta
+from typing import Optional, Sequence
 
 from django.apps import apps
 from django.db import models
@@ -9,6 +9,18 @@ def current_week() -> int:
     return int(datetime.now().strftime('%V'))
 
 
+def week_days(week: Optional[int]) -> Sequence[date]:
+    # FIXME Make this aware of the school term concept
+    # cf. BiscuIT-ng#40
+
+    year = date.today().year
+    wanted_week = week or current_week()
+
+    first_day = datetime.strptime('%d-%d-1' % (year, wanted_week), '%Y-%W-%w')
+
+    return [(first_day + timedelta(days=offset)).date() for offset in range(0, 7)]
+
+
 def current_lesson_periods(when: Optional[datetime] = None) -> models.query.QuerySet:
     now = when or datetime.now()