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

Neat, but I would expect that we're going to trash this with the new webservice, and I wouldn't put too much effort into it :p
But then again, unkept software tends to stick around...

I'm also expecting it will be thrown away at some point, and that's why I only did a fairly minor refactoring. But I imagine it will be in use for a while yet, and making it a bit clearer what it's doing might also be useful for developing the replacement.

changed milestone to %3.5.0

added Waiting for review label

@roscar Could you please have a brief look here? Now with the work done, we should benefit from it!

LGTM

Thanks!

removed Waiting for review label

mentioned in commit a94ff6b0

merged