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.
I would use "item" rather than "name" here. We frequently use "first_name" and "last_name" to refer to a person's name (eg "Renzo", "Lucioni") and this is confusing. "Organization name" is also a bit ambiguious without an example. I'd make this more explicit:
```python
# Translators: The join of three or more organization names, eg, "MIT, Harvard, and Dartmouth"
_("{first_items}, and {last_item})
```

Also, the ugettext function ("unicode gettext") always returns a unicode object, so putting the `u""` is unnecessary.