Login Redirect Workflow

We want to handle redirected the user properly when they are viewing a resource they are unable to. The current implementation, merged in in PR #59, redirects any unauthorized users to the home page (also the login page).

- [ ] Indicate to user why they are being redirected #60
- [x] Encode where user came from #61 so we can redirect them
- [ ] Handle not logged in vs unauthorized properly #62
- [ ] Add auth decorators everywhere #9

Review PR #59 and the linked issues #60, #61, #62, and #9 to understand the existing redirect workflow and planned authentication behavior. Done means the issue's listed redirect, authorization, login-state, and decorator requirements are resolved without sending every unauthorized user to the home or login page.

Written by the indexing model from the issue text.