Move threat model doc to GitHub

Move threat model doc to GitHub
It's generally good to provide a meaningful structure of nested headers. This allows us to auto-generate TOC for these documents, for example. Jumping from Heading Level 1 to Heading Level 5 does not make sense. I would recommend a structure similar to:

```
# SecureDrop Threat Model
## Assumptions
### Assumptions about the Source
### Assumptions about the Admin and the Journalist
...
## Attack Scenarios
### ...
```