Documents hosted using Agency API domain by default

Proposal and estimate documents will now be hosted using the agency API domain by default along with the support for invoices/sales receipts
This will by default host all the current as well as new documents created using the API domain configured at the agency level
If the agency API domain is not present, the documents will fall back and hosted using the
sendlink.co
domain as default support
This gives more control to agency owners over the domains used for hosting any proposal/payments features