Use pendulum for datetime operations when possible

Currently, we use both Python's built-in datetime library and Pendulum
to do datetime operations. For the sake of consistency, we should try to
stick to one library for datetimes. We could have used either, but
Pendulum has a more ergonomic API, so I decided to go with it when
possible.

The places where were we didn't / couldn't replace datetime are:
- checking instances of datetimes. Pendulum's objects are subclasses of
  python native datetime objects, so it's still useful to import
  datetime in those cases of using is_instance()
- WTForms date validators expect datetime style string formats --
  Pendulum has its own format for formatting/ parsing strings. As such,
  our custom validator DateRange needs to use datetime.stptime() to
  account for this format.

script/seed_sample.py

@@ -1,7 +1,7 @@
 # Add root application dir to the python path
 import os
 import sys
-from datetime import timedelta, date
+import pendulum
 import random
 from faker import Faker
 from werkzeug.datastructures import FileStorage
@@ -170,10 +170,9 @@ def add_members_to_portfolio(portfolio):
 
 
 def add_task_orders_to_portfolio(portfolio):
-    today = date.today()
-    future = today + timedelta(days=100)
-    yesterday = today - timedelta(days=1)
-    five_days = timedelta(days=5)
+    today = pendulum.today()
+    future = today.add(days=100)
+    yesterday = today.subtract(days=1)
 
     def build_pdf():
         return {"filename": "sample_task_order.pdf", "object_name": str(uuid4())}
@@ -192,13 +191,13 @@ def add_task_orders_to_portfolio(portfolio):
 
     clins = [
         CLINFactory.build(
-            task_order=unsigned_to, start_date=(today - five_days), end_date=today
+            task_order=unsigned_to, start_date=today.subtract(days=5), end_date=today
         ),
         CLINFactory.build(
-            task_order=upcoming_to, start_date=(today + five_days), end_date=future
+            task_order=upcoming_to, start_date=today.add(days=5), end_date=future
         ),
         CLINFactory.build(
-            task_order=expired_to, start_date=(today - five_days), end_date=yesterday
+            task_order=expired_to, start_date=today.subtract(days=5), end_date=yesterday
         ),
         CLINFactory.build(
             task_order=active_to,