Pswadi/login restriction

Added an environment switch to restrict login to emails to company domain.

/cc @zendesk/samson 

### References
 - Jira link:

### Risks
 - None
Don't think we should restrict github users. We already verify that they are in a certain organisation. Github users don't get a choice of what of their registered email addresses they use. I'd prefer this just stays with google auth for now.