Put issuable owner and participating people first on mention suggestions

Closes #14423

**Merge Requests**

Author and people who participated are first now


<img src="/uploads/dd75937627c61b5efe631b4a35f274c6/Screen_Shot_2016-03-29_at_6.41.29_PM.png" width="235" />

**Issues**

Author showing first and no people have participated

<img src="/uploads/e17aa96ffe7d46a7dbe293318e999e1d/Screen_Shot_2016-03-29_at_6.41.44_PM.png" width="279" />


See merge request !3448