Refactor serve_overview code a bit

Description

Some fairly simple changes to make the serve_overview code easier to follow:

• Pull out code for serving separate pages & files into their own methods.
• Remove the unused pdf_queue mechanism and pdf?<uuid> endpoint - this hasn't been used since February 2020 (61f9d099).
• Send a 404 when we can't send a static file, rather than an OK response with no body.
• Send Content-Length header for static files - this gives nicer feedback in the browser when e.g. opening a large PDF.

How Has This Been Tested?

Installed on max-exfl017, checked the overview page, darks overview, and opening PDF files.

Types of changes

• Bug fix
• Refactor

Checklist:

• My code follows the code style of this project.

Reviewers

@ahmedk @roscar