Update copy used in the checkbox label

Updating the copy used in the email opt-in checkbox label as per [ECOM-687](https://openedx.atlassian.net/browse/ECOM-687). This copy is pending approval from @griffresch.

@dianakhuang, could you take a quick look? This just changes the value of a string, so I don't think two reviewers are needed. I'll merge once I get sign-off from Griff.
The proper way is to use 4 internationalization strings:
```
# Translators: the join of two elements (eg, "Saturday and Sunday")
_({item1} and {item2})
# Translators: The separator between the first two of a 3+ element list: eg, in "Saturday**,** Sunday, and Monday"
_({first_item}, {remaining_items})
# Translators: The separator between the middle of a 4+ element list: eg,  "Saturday, Sunday**,** Monday, and Tuesday"
_({first_items}, {remaining_items})
# Translators: the separator between the end of a 3+ element list: eg, "Saturday, Sunday**, and** Monday"
_({first_items}, and {last_item})
```

Taken rather verbatim from http://cldr.unicode.org/translation/lists. We can't make everything perfect, so somehow incorporating this into the translation would be good. Perhaps skipping the 3rd form because incorporating that will be tricky to code and not sure worth it, not sure if there'd _really_ be a difference between the 2nd and 3rd form.