Add Django Pattern Library

[dchukhin]

One-line summary

This pull request adds the django-pattern-library to the project. Note: the official django-pattern-library may not support Jinja2, so we use LL's fork.

Significant changes and points to review

• the django-pattern-library should be available at the /pattern-library/ URL (no language in URL)
• the django-pattern-library should only be available in environments that set the ENABLE_DJANGO_PATTERN_LIBRARY environment variable to True
• this pull request provides 1 example: button.html, but many more examples will be created in future pull requests

Issue / Bugzilla link

Testing

┆Issue is synchronized with this Jira Task
┆Priority: P3
┆Sprint: Backlog

[maribedran]

Pattern library is already in