[cloud] Case insensitive comparison of emails for invitations

Created by: kopancek

Previously, there was a case sensitive string comparison of the email on the invitation with the emails configured on the user account. But email is case insensitive in it's nature, meaning that people are likely to fill their email as [email protected] instead of [email protected]

Doing a case insensitive comparison fixes the problem.

Related: https://sourcegraph.atlassian.net/browse/CLOUD-337

Test plan

Tested with unit test