Remove has_many `_ids` suffix from form labels

Created by: gracewashere

Fixes #303 (closed)

Problem:

When HasMany attributes are displayed in form fields, their labels include the _ids suffix that Rails uses internally (e.g. line_item_ids, order_ids).

Users expect to see the pluralized form of the relationship name (e.g. line_items, orders).

Solution:

Pass in the desired label value explicitly instead of letting Rails guess at the correct label.