Currently, we're using values from 0 to infinity in the URL parameters for pre-selecting a bucket. A value of "0" can lead to problems in mass mailing software, which converts 0 to an empty string.

Acceptance criteria:
Instead of numeric values in the URL, use alphabetic values (i.e. bucket names).

Determine if we need to have backward compatibility to existing URLs.

Implementation details:
The only change should be in BucketSelectionServiceProvider, because the rest of the code (and the cookie values) should stay the same.