[Admin] Promo code batch creation with email field

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In the admin there are 2 ways for creating promotion code batches (Spree::PromotionCodeBatch):

1. from the new promotions page at the bottom of this partial;
2. from the edit promotion page, by following the links in the header Promotion Code Batches -> New Promotion Code Batch, see this form

The second way allows to specify an email address where a confirmation email will be sent after the batch have been processed. This does not happen with the first flow.

For consistency, I think we should have the email field also on the first path. Also, if possible I think it would make sense to unify the erb code that handles both pages, so we won't happen to possibly have different behaviors again in the future.

Solidus Version:
master and below

Screenshots

first path	second path