Taiga #831 Bug 15774 - Ability to add additional fields for baskets

Taiga #831 Bug 15774 - Ability to add additional fields for baskets

This new feature in 19.05 added the ability to define additional fields to store information about acquisition baskets.

It refactors and centralises the code used to add fields to subscriptions as well. There is a new 'Additional fields' page in the administration module to configure the fields.

Users can name additional fields, tie them to authorised values, and specify whether the fields can be searched in the acquisitions module.