This paves the way for custom benefits to return successful applications
that don't lead to a discount.

This might be futile.

* Includes a migration for the proxy_class field
* Dashboard templates needed adjusting

Fixes #386

Renaming column for clarity now that we have alternative fields for
tracking consumption counts

Fixes #376

Added new field to offer that allows their max num applications to be
controlled.  Dashboard adjusted to support this too.