Fix unread-remove-deleted-messages script so it actually removes stuck unreads

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• Fix unread-remove-deleted-messages utility script so it actually runs and removes stuck unreads
• Refactor to use some async/await syntax
• Change was made to address stuck unread situation here, ☝ September 25, 2018 3:25 PM

Run locally on Windows:

$ mklink "C:\Users\MLM\Documents\GitLab\gitter-secrets\webapp\env" "C:\Users\MLM\Documents\GitLab\gitter-secrets\env"
$ @FOR /f "tokens=*" %i IN ('bash /c/Users/MLM/Documents/GitLab/gitter-secrets/webapp/env prod') DO @%i
$ set NODE_ENV=prod&&node ./scripts/utils/unread-remove-deleted-messages.js --uri gitterHQ/gitter --dryRun
$ set NODE_ENV=prod&&node ./scripts/utils/unread-remove-deleted-messages.js --uri gitterHQ/gitter