Add favicon.ico to Django static files

[Investigamer]

Description

Fix crash caused by missing favicon.ico static file.

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate.
• I have updated any relevant documentation or created new documentation where appropriate.

[ndepaola]

• the frontend relies on the server having a favicon.ico file for the second step of the server connection workflow (using ping.js):
  

• if the system is deployed like you've configured it, where both the frontend and backend are hosted in the same domain, and any requests on /2/ or /admin/ are reverse proxied to django but other requests are reverse proxied to the frontend, then the frontend's favicon.ico will answer the ping request

• however, if the backend is deployed headlessly (without a frontend) and you try to connect to it through https://mpcautofill.github.io, i expect the ping step will fail because the backend has no favicon.ico

• so i reckon the correct change here is to revert the deletion of favicon.ico from Delete old frontend #176

[Investigamer]

Done! Looks all set 👍