Add bulk delete feature for promotion entries

Problem/Motivation

Given that entries for promotions can be numerous, and given that some users may wish to delete multiple entries, the ability to bulk delete entries might be a useful addition to the interface at /admin/content/vais_promo

Proposed resolution

The table at /admin/content/vais_promo is updated to allow for multiple entries to be selected and then deleted

User interface changes

1. Checkboxes to select multiple entries
2. A delete button
3. Delete confirmation page