Improved support for sub-directory deployment behind proxies

When you deploy FeedHQ under a sub-directory (e.g., `/feedhq/`), you may end up with invalid URLs for all media/static files, and encounter some dead links. Consequently, I added two new environment variables: `MEDIA_URL` and `STATIC_URL`. This allows any deployment to serve assets from any location (e.g., `/feedhq/static/`, `https://assets.example.com/static/`). I also replaced hardcoded links by their equivalent with `reverse` and `url`.