cms: Add a warning for admins for multiple invoices for a user in the same month/year

[alexlyp]

If there are invoices by a user that share the same month/year then there should be a warning shown to the admin.

[xaur]

Such warning could be shown at the top of each invoice with same month/year, ideally with links to jump to the others, e.g.

There are other invoices for same month and year: [c37ad21 ($500)](link...), [15ed87a ($0)](link...)

Also, invoices with same month/year are sometimes created by mistake and need a way to be revoked by the user - related feature request #2847