Add function to estimate letter delivery date

Letter delivery depends on:
- how long it takes to print
- how long it takes to post

Both of these process are impacted by weekends, because people don’t
work on weekends.

It also depends on if you submit your letter before or after 5pm,
because that’s the cut off time for getting a letter printed on a given
day – ie after 5pm on Monday is effectively the same as Tuesday and so
on.

But I reckon all our users need to know is roughly how long it will take
until the letter turns up on the user’s doorstep. So this commit adds
a function to calculate this. Doesn’t surface it on the front end _yet_.

app/utils.py

@@ -1,10 +1,12 @@
 import re
 import csv
+import pytz
 from io import StringIO
 from os import path
 from functools import wraps
 import unicodedata
 from datetime import datetime, timedelta, timezone
+from dateutil import parser
 
 import dateutil
 import ago
@@ -324,3 +326,25 @@ def get_time_left(created_at):
 
 def email_or_sms_not_enabled(template_type, permissions):
     return (template_type in ['email', 'sms']) and (template_type not in permissions)
+
+
+def get_estimated_delivery_date_for_letters(upload_time):
+
+    # shift anything after 5pm to the next day
+    processing_day = gmt_timezones(upload_time) + timedelta(hours=(7))
+
+    return tuple(
+        processing_day + timedelta(days=days)
+        for days in {
+            'Wednesday': (3, 5),
+            'Thursday': (4, 5),
+            'Friday': (5, 6),
+            'Saturday': (4, 5),
+        }.get(processing_day.strftime('%A'), (3, 4))
+    )
+
+
+def gmt_timezones(date):
+    date = dateutil.parser.parse(date)
+    forced_utc = date.replace(tzinfo=pytz.utc)
+    return forced_utc.astimezone(pytz.timezone('Europe/London'))