Drop dependency to jinja2

[SebCorbin]

Reviewing https://github.com/jazzband/django-silk/pull/623 I noticed we are using jinja2 only twice for classic templating:

• https://github.com/jazzband/django-silk/blob/8a8ac08282bcc985ad86c7a4bf28a25b1c3dcde5/silk/code_generation/curl.py
• https://github.com/jazzband/django-silk/blob/b6c3934ff606db0a208cd4c7f0461f6f7ea2787a/silk/code_generation/django_test_client.py

We could keep the jinja2 package dependency out by using django template instead.