Add method to revoke token allowance

Many exchanges and sites involve granting a token allowance, which leaves the user's token balance exposed to the delegate contract.

As the user's wallet, we should [make it easy to revoke any permission that we make it possible to grant](http://zesty.ca/pubs/icics-2002-uidss.pdf).

Where should we put this?

Our current permissions system is largely per-site, but allowances are per-contract, so while it may make a lot of sense to show the allowance in-line with a site's permissions, it isn't a perfect representation.

We may also want to show outstanding allowances in the token's view.

The issue does not name files, tests, or entry points. Start by reviewing the current permissions system and the token view, then determine where contract-level allowances belong; done means users can identify and revoke allowances that the wallet can grant.

Written by the indexing model from the issue text.