Cert warning on upload

[matthugs]

A user reported this warning in the console upon uploading emoji:

The SSL certificate used to load resources from https://slack.com will be distrusted in M70. Once distrusted, users will be prevented from loading these resources. See https://g.co/chrome/symantecpkicerts for more information.

We should investigate and see whether this distrusting would be likely to jeopardize the smooth operation of the script. If not, it should be documented in FAQ documentation.