Expand FAQs

Subtask of #28

- What is Briar?
- How does Briar work?
- Why is it called Briar?
- How is Briar different from other messaging apps?
- How do I install/update/uninstall Briar?
- What platforms can I use Briar on?
- Can I send files and images? 
- Can I do video calls?
- Why is Briar is always logged out every time I open the app?
- What is the difference between a forum, private group and blog?
- What does Briar use for end2end encryption
- How are messages delivered?
- Why is my connection to a private group so slow?
- What happens to abandoned accounts?
- Why does Briar drain more battery?
- What are some limitations of Briar?
- how can briar sync over the internet without a server? how do you know where to send the sync to? don't you need some kind of known address or middleman to sync?