Text changes - Consistency & Clarity - Gift Vouchers and (customer) Transactions

Removal of the "Gift Certificate" and "Voucher" terminology everywhere in favor of "Gift Voucher". This wasn't really based on my preference for voucher. I just wanted us to say one thing or the other and stick to it, i.e. be consistent, not mix the two...certificate just drew the short straw. I'm equally fine with Gift Certificate. But if we call it Gift Certificate then let's call it Gift Certificate throughout.

Renaming of customer "Transactions" to "Store Credits"... in the original (unpatched) version we refer to these as store credits in some places but transactions in others...also transactions is more ambiguous...one might expect, for example, to see payments made for orders to appear under transactions which is clearly not the case.

There might be a couple of other small changes as Github doesn't let me pick out specific lines of files to include in patch

Sorry, I remembered too late that you can't merge separate commits within a pull request separately. Thankfully, I had only submitted 2 small commits, so submitting them as one PR. From next commit on, I'll submit one PR per commit.