Require login to access uploaded images #100

currently, images uploaded to DAV have public urls that can be access from anywhere.

Potential solutions

1. add referrer checking (spoofable)
2. proxy images through the forum itself (inefficient if using remote image host)
3. pluggable auth

• local is likely easy, but can always proxy through forum
• remote is a lot harder, may need to consider a real auth provider at some point