LP#1748986 Broaden billing interface labels

LP#1748986 Broaden billing interface labels

In a few places we use the word "Paid", but there are quite a few
things in that bucket (forgiveness, adjustments, etc.) which don't fit
well under the term "Paid".

Though we have some deep structural causes for this, we can take a baby
step in the right direction by sprinkling the term "Credited" in a few
key places.

Also, correct an mislabeled "Balance" field.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>