batches: select all on the preview page and publishing fails

Created by: LawnGnome

This was reported in https://github.com/sourcegraph/customer/issues/812.

Reproduction:

1. Run src batch preview with a spec that generates more than 15 changesets.
2. Go to the preview link.
3. Select all changesets, then click the (Select all XX) link that appears.
4. Choose a bulk action (it doesn't matter which one), such as Unpublish on apply.
5. Click the blue button.

You get this error:

2 errors occurred: * malformed changeset spec ID "a": illegal base64 data at input byte 0 * malformed changeset spec ID "l": illegal base64 data at input byte 0

It looks like the literal "all" that is supposed to be special cased doesn't get handled properly.