[Receipts List/Admin] - Add a comment field for expense rejection

[AleHundred]

When reviewing expenses, some may be marked invalid for reasons that are not necessarily tied to their legality, so it would be a more precise user experience to remove that wording to avoid confusion as well as adding a new field so that the uploader knows why it was rejected.

• Remove the word "legally" from "Mark legally valid/invalid" buttons
• Add a comment field so the expense uploader knows why it was marked invalid if that's the case.

[chmac]

@AlxSavage The idea was that we would check to make sure that the invoice meets all the German bureaucratic requirements. I like the concept of a comment to explain why, that's a neat idea.

[chmac]

PS> I'm not sure I'd remove the label from the legally valid part. I guess we could also add another type of rejection if that was necessary. Or maybe that's overcomplicating, not sure! :-)