E-Accounting: Store Equal ID instead of Number in getPricingsForAdministration

Saves extra requests to retrieve the external ID of a rate when passing an invoice and then it is in line with the other links. I see it does work fine for clients and projects.

Maybe just check for all links and all get...OptionsForAdministration that we are doing this correctly and take out the extra (then redundant) requests.

Also needs a migration for the current situation.