[WIP] Use Twig to render all HTML

Please don't merge this in yet, it isn't ready. I'll remove the WIP tag when it
is.

This is being made with the intent of closing #31 and making #32 a possibility.

Tasks:

- [ ] First Mention tool
- [ ] Following Comparison tool
- [ ] Signin
- [ ] Signout
- [x] Homepage
- [x] About page
- [x] Donate page
- [x] Terms page
- [x] Privacy page
- [x] User lookup page
- [x] Broadcast tools overview page
- [x] Broadcast Channel Lookup tool