No easy way to abort form

Problem/Motivation

When using the Convert Bundle tab on a node, it stores this NID in a private temp store.
Then every time we get back to the form, it remembers this node.
There is no easy way to abort.

(Browsing to /admin/config/content/convert_bundles clear the temp store but it's not very intuitive.)

Steps to reproduce

Browse to /node/42/convertbundles.
Leave the form.
Browse back to /admin/convert_bundles: you are still converting this node.

Proposed resolution

We could add a Cancel button on the form.

Remaining tasks

User interface changes

API changes

Data model changes