Zms 86 mailboxes

[NickOvt]

• tests for POST /users/{user}/mailboxes
• tests for GET /users/{user}/mailboxes
• tests for GET /users/:user/mailboxes/:mailbox
• tests for PUT /users/:user/mailboxes/:mailbox
• tests for DELETE /users/:user/mailboxes/:mailbox

[andris9]

Does the in progress and TODO mean that you will be updating this PR with additional tests or is it done and you add these tests later or in a separate PR?

[NickOvt]

Yes, will add tests for other 3 endpoints under this PR as well, just different commits.