feat: support configuring the base path

[rgelinas-polygon]

Make sure these boxes are checked! 📦✅

• You ran ./run_tests.sh
• You ran pre-commit run -a
• If you want to add your network to setup_service.py, provide a link to your
  safe-deployments PR and check network name
  exists in safe-eth-py

What was wrong? 👾

We are looking to proxy many transaction services behind a single domain - because there are redirects (swagger functionality, Django appending slashes and redirecting by default, etc.), we need to change the base path of the app. We experimented with the FORCE_SCRIPT_NAME option added in #400, but it did not solve the problem for us. Changing the script name adjusted the base domain rather than solely the base path Django expects to use.

How was it fixed? 🎯

I simply added a new environment variable called DJANGO_BASE_PATH that fills out the BASE_PATH setting, and ensured that all URLs were prefixed by the base path. By default the base path is empty, and nothing changes.

[github-actions]

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[rgelinas-polygon]

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[moisses89]

@rgelinas-polygon could you try configuring the prefix midleware header from your proxy?
#2556