User ID extractor for handlers that require an user id

[mario-nt]

Parent issue: #39.

Tasks:

New extractor

• User Id extractor (for handlers that require an user id)

Status code and error

• Custom required logged-in user error

Refactor handlers

• torrent upload torrent
• torrent update torrent info
• torrent delete torrent
• category add handler
• category delete handler
• settings get all handler
• tag add handler
• tag delete handler
• user ban user handler
  - [ ] proxy get proxy handler