Acq: When creating invoices, let the user know if they're repeating an ID

Acq: When creating invoices, let the user know if they're repeating an ID

Invoices have a field for Vendor Invoice ID, which is required to be
unique for invoices associated with a given provider (vendor).  In
practice users may mistakenly try to create an invoice more than once, so
this gives them an indication that a given Vnedor Invoice ID has already
been used before they fill out the whole invoice.

Users do have to have the fields labeled "Vendor Invoice ID",
"Provider", and "Receiver" filled out before validation can occur (the
last field is needed because of how permissions are checked when
searching for invoices).

It is still possible to see a database error at save time if another
invoice is created between the time you started yours and the time
somebody else saved theirs.

Also, while we're at it, this commit gets rid of acq/invoice/common.js
and moves its last function into acq/invoice/view.js, since both are
actually used only by the one interface.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>