Bump django-sitetree from 1.17.0 to 1.18.0

Bumps [django-sitetree](https://github.com/idlesign/django-sitetree) from 1.17.0 to 1.18.0.
- [Changelog](https://github.com/idlesign/django-sitetree/blob/master/CHANGELOG)
- [Commits](idlesign/django-sitetree@v1.17.0...v1.18.0)

---
updated-dependencies:
- dependency-name: django-sitetree
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

base-requirements.txt

@@ -1,6 +1,6 @@
 dj-database-url==0.5.0
 django-pipeline==2.0.6
-django-sitetree==1.17.0
+django-sitetree==1.18.0
 django-apptemplates==1.5
 django-admin-interface==0.24.2
 django-translation-aliases==0.1.0