Negative option was leading to a fair amount of confusion, going forward
if we want to allow selection of emails from user selector it must be
supplied with `allowEmails=true`
This corrects a regression in 1f4ace4f which broke invite by emails and
start PM to email
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>
```
Previously we were incorrectly checking mentionable instead of messageable
Also fix edge case where multiple groups sharing a name mean that exact match override is not working
Also cleans up params sent to user selector
