Following this change when a user hits `@` and is replying to a topic they
will see usernames of people who were last seen and participated in the topic
This is somewhat experimental, we may tweak this, or make it optional.
Also, a regression in a423a938 where hitting TAB would eat a post you were writing:
Eg this would eat a post:
``` text
@hello, testing 123 <tab>
```