Site-wide notifications and login view

- Adds site-wide notifications. Well, sort've. Due to the styling of the site, it's difficult to get a notification block in `base.html` while also looking correct. Instead of refactoring a bunch of CSS and HTML to make this happen, I just made the notices into a macro and used it on the major pages. A side benefit of this is that it's easier for us to choose which pages should not have notifications on them.
- Adds a small login view that redirects to the homepage and adds a notification alerting the user that they need to log in. Not really related to notifications, but it depended on them and is tiny.