[keyserver] Fix emojis in MySQL backups

Summary:
When I tested restoring my keyserver backup on my laptop, I noticed that all of the emojis were broken. I did some Googling and found some results that together pointed at this solution.

Depends on D4176

Test Plan: Backup my keyserver DB on server and then restore on my laptop

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4177