Review use of mizzao/meteor-autocomplete

[engelgabriel]

Check if we still need the package copied into our repo or can we use the latest from https://github.com/mizzao/meteor-autocomplete or should we just turn into our own?

[simonv3]

This package hasn't been updated in a year, and I feel like the autocomplete functionality in RocketChat isn't that good, so I would argue for either using a different package that does auto-complete a bit better or implementing it ourselves.

[timkinnane]

Relevant #10872 - if the package gets reviewed, I think it would be a good opportunity to address long standing experience issues with username autocompletion.