[WIP] Reworked ability to edit invoices, as well as many fixes and smaller changes.

[Anuril]

This draft PR is part of a larger overhaul of the invoicing of Fossbilling.

Here is a list of current changes:

• Changed the Invoice Detail view in the Admin area to be more condensed
  

• Removed the ability to edit most of the invoice details.

• Modifying invoice date, due date and paid date is only possible as long as the invoice is unpaid.
  

• Added the ability to refresh the invoice's client data from the client record
  

• Invoices can't be deleted anymore if the client had the possibility to see it (Meaning: The invoice has the state of "approved")

• Instead, invoices can be revoked if they're unpaid. Paid, refunded, cancelled or revoked Invoices can't be deleted.

• If an unpaid invoice isn't approved, it can be deleted just as before.

• Revoking an Invoice removes it from the associated order, making the order "available" again for manual or automated renewal.
  

• Added an Archive tab to the invoice list to quickly see revoked Invoices. (And so they don't show up in the invoice list)
  

• Fixed the Missing Price field in the Invoice item list, changed formatting so it's more consistent
  

• Changed colors (and batch formats) for invoice states so they are consistent between admin and client interfaces
  

• As most helpfully implemented by @gilesytheking in Add "Due At" Column on Admin Invoice Overview #1992, I've changed the due date / paid date fields in the tables, as well as changing many smaller inconsistencies around format_date
  
  

• Added "Refunded" and "Revoked" tabs to the client Invoice overview (Only displayed if the client has refunded or revoked invoices)
  

• Changed the wording for most Client-Side Things around refunded Invoices to "Credit Note"
  

Todos

• Filtering for invoices is broken unless and until a Client is selected.
  
  

• VAT % and Amounts are not stored in the invoice. invoice item price is the same as total in the database. Currently, it's a workaround that calculates the values in Situ, which has to be fixed.

• For the Revoke functionality, no tests exist yet, and while I'm at it, I'll go over the other existing tests for the Invoice Module