Use utils template to format broadcast content

Brings in:
- [x] https://github.com/alphagov/notifications-utils/pull/801

Formats the content of the template at the time of creating the event.
This means that any downstream code (eg Celery tasks) can assume the
content is already formatted correctly.

Also, these downstream tasks don’t  know which template the broadcast
was created from, so if we support personalisation in the future this is
the most sensible place to bring together the template and the
personalisation.

---

I had to re-create some of the deleted code from utils for stuff like
formatting the timestamp to the CAP standard.

requirements.txt

@@ -31,7 +31,7 @@ notifications-python-client==5.7.0
 # PaaS
 awscli-cwlogs>=1.4,<1.5
 
-git+https://github.com/alphagov/notifications-utils.git@42.2.1#egg=notifications-utils==42.2.1
+git+https://github.com/alphagov/notifications-utils.git@43.0.0#egg=notifications-utils==43.0.0
 
 # gds-metrics requires prometheseus 0.2.0, override that requirement as 0.7.1 brings significant performance gains
 prometheus-client==0.8.0
@@ -42,14 +42,14 @@ alembic==1.4.3
 amqp==1.4.9
 anyjson==0.3.3
 attrs==20.2.0
-awscli==1.18.148
+awscli==1.18.164
 bcrypt==3.2.0
 billiard==3.3.0.23
 bleach==3.1.4
 blinker==1.4
 boto==2.49.0
 boto3==1.10.38
-botocore==1.18.7
+botocore==1.19.4
 certifi==2020.6.20
 chardet==3.0.4
 click==7.1.2
@@ -66,7 +66,6 @@ importlib-metadata==2.0.0
 Jinja2==2.11.2
 jmespath==0.10.0
 kombu==3.0.37
-lxml==4.5.1
 Mako==1.1.3
 MarkupSafe==1.1.1
 mistune==0.8.4
@@ -89,7 +88,7 @@ s3transfer==0.3.3
 six==1.15.0
 smartypants==2.0.1
 statsd==3.3.0
-urllib3==1.25.10
+urllib3==1.25.11
 webencodings==0.5.1
 Werkzeug==1.0.1
-zipp==3.2.0
+zipp==3.4.0