Fix path traversal vulnerability in the pfp feature

[mataiodoxion]

Read about the exploit here (I know I'm publishing this before the PR is actually accepted, but there's a very small window of opportunity): https://mataiodoxion.github.io/blog/smashing-morts-server-1/

TL;DR Most notably, I could have escalated my permissions on the site to admin by exfiltrating .env secrets, logging into the admin server, and making some queries. I did, however, escalate my privileges with the RCE (through the code runner) a bit earlier.

Updated two files to use a safer version of os.path.join() with secure_filename() at both the fetch level and interal setter level. Main idea is that 1) inputs weren't properly sanitized and 2) os.path.join() will take an absolute path over a local path, meaning I could set that to any file I wanted on the Docker image (including .env, of course).

PS: My IDE's built in linter automatically formatted a bunch of the code, so some of the code format styling changed a bit.

Notable lines:

• model/pfp.py: linter didn't mess with the additions too much here
• model/user.py: 348-364