When tagging people, prioritize teammates over exact username matches (BB-9462)

I often need to tag teammates in issue comments to gather their input but I often end up tagging random people due to the lack of prioritization when it comes to teammate tagging.

For example:

One of our project managers, Kelaine, has the tag @kelaine but if I get to @kel and press enter, it tags "Kel" (another user apparently) instead of auto-filling the rest of her name. I very rarely (never actually) have to tag people outside of my org. in our private repos so either defaulting to teammates first or allowing only teammate autocomplete in org repos would be very helpful.