cms: create batched calls for invoices

[fernandoabolafio]

The batched endpoints allowed a much simpler way of fetching proposals as needed in the politeiagui side.
Since we also always display invoices grouped by their status, having a single endpoint creates an inconsistent pagination mechanism and requires different handling in the client.
Having batched calls for invoices will increase the code consistency in the frontend side, speed up the loading and provide a better lazy load solution. That said, two accomplish that in the CMS side we need two add the following calls:

• request token inventory for user invoices
  • POST call
  • params:
    • timestampMax
    • timestampMin
• request token inventory for admin invoices
  • POST call
  • params:
    • timestampMax
    • timestampMin
• request batch of invoices
  • POST call
  • params:
    • an array of invoice tokens

The current invoice statuses we have are:

• unreviewed
• updated
• disputed
• approved
• paid
• rejected

The main difference from what we currently do for proposals is the addition of the params timestampMax and timestampMin when asking for the token inventory. The CMS requires the invoices to be filtered by month so to accomplish the desired output here we need to know beforehand what invoices to fetch based on the time filter.

[thi4go]

Would like to work on this.

[alexlyp]

ok cool, feel free to ask for any help that you may need.

[victorgcramos]

@alexlyp what about the same for DCCs?

[alexlyp]

Yup makes sense to add those as well. Though different PR obvi

[victorgcramos]

Sure! of course! After I finish other stuff on GUI, I'd like to jump on this.