Add other possible Mozilla internal emails to firestore.rules

[brizental]

Currently firestore.rules allow mozilla.com and mozillafoundation.org emails. With more Mozilla products using Glean we might want to be a step ahead and also include emails such as getpocket.com and thunderbird.net in there.

This issue is for voming up with a final list of emails we might want to include and to implement the new rules.

See initial discussion: #37 (comment)