Urls and pagination ignore api version

#### Issue description
The projects API returns errornous API links in the pagination section (`next`, `previous`) and in `results[*].url`.

#### Steps to reproduce the issue

1. Navigate to https://fixmyberlin.de/api/v1/projects
2. Inspect the `next` field in the response json

#### What's the expected result?

- `"https://fixmyberlin.de/api/v1/projects?page=2"`
- Following this link brings me to the next page of the list

#### What's the actual result?

- `"https://fixmyberlin.de/api/projects?page=2"`
- Follwing this link brings me to the API landing page

Start by running the documented request against https://fixmyberlin.de/api/v1/projects and inspect the pagination next/previous fields and each results[*].url value. Trace the projects API entry point and URL-generation or pagination configuration; done means every generated link preserves /api/v1 and the next-page link returns the following project list rather than the API landing page.

Written by the indexing model from the issue text.