Allow the use of Service Desk custom mailbox without a suffix

Using a custom email address for Service Desk currently requires configuring a custom mailbox and a custom suffix.

Without the custom suffix, the worker cannot identify the project in which to create an issue.

This is confusing for users as they can configure a custom mailbox but not use it without first going to project settings and setting a suffix. On GitLab.com, the incoming email address will be shown until the suffix is set, and will revert if the suffix is removed. This is also confusing and unexpected.

Proposal

Update the worker to ignore the suffix entirely (if it's not set) and find the project from the group and project id in the key. This would allow the user to customize the Service Desk email address without having to add an additional suffix if they don't wish.