client needs to not check up on twitter proofs users have totally bailed on OR front end needs button to remove

@adamjspooner writes:

> Here’s what I see: https://cloudup.com/cCZt0kxQmmN
>
> Here’s what I did:
> 1. began Twitter proof
> 2. bailed on the process because I didn’t want to tweet something (I don’t use Twitter anymore)
>
> ```
> ```

In theory this looks fine (the twitter proof is long gone) but the client fails to id adam, because it tries to prove his twitter identity too.

If someone's proof is not yet complete, one of these should be happening:
- the client needs to ignore it
- the front end needs to make them cancel it

(or both)

Start by reproducing the abandoned Twitter proof flow and inspect how the client handles incomplete proofs and how the front end could cancel them. Done means an incomplete, abandoned proof no longer prevents identifying the user, with a cancellation option if that path is retained.

Written by the indexing model from the issue text.