Tracking issue: inline TODO comments

[LukasKalbertodt]

There are quite a few // TODO comments right now. While eventually all of those should be removed and converted into GH issues, right now it's still handy to have those around. However, I'll try to list them here to avoid forgetting an issue.

• [ ] README.md: Write a better summary
• [ ] build.rs: use Rocket's environment
• bin/manage_db.rs: find user by username #35
• [ ] db/mod.rs: maybe tweak r2d2 config if necessary
• [ ] db/mod.rs: maybe install another error handler for db pool
• [x] login/mod.rs: reactivate "Associations" feature of diesel
• login/routes.rs: redirect to original request path #34
• [x] template/mod.rs: Unhide help on login page?

[LukasKalbertodt]

From phase 1:

• [ ] login/html.rs: "this hint is not really correct. The hint depends the chosen login-provider. So we have to adjust the hints dynamically. We should. In the future, probably."
• login/ldap.rs: impl #25
• [ ] login/password.rs: use joins to avoid second query, probably
• [ ] login/routes.rs: give developer the ability to login with internal password in prod mode
• user/settings.rs: implement #27