[Security] Bump django from 2.1.11 to 2.1.15 in /backend/uclapi
[Security] Bump django from 2.1.11 to 2.1.15 in /backend/uclapi

Bumps [django](https://github.com/django/django) from 2.1.11 to 2.1.15. **This update includes a security fix.**
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.1.11...2.1.15)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
uclapi/uclapi,uclapi/uclapi,uclapi/uclapi,uclapi/uclapi