Better cleanup of webwork_post_data option values

[boonebgorges]

Previously: #193

A few ideas:

• Store this data in a transient, which is automatically purged, rather than an option. In most cases it's needed only for a few minutes, while the redirect (and perhaps the login flow) takes place
• Store the timestamp with the post data, and then perform regular cleanup routines of anything older than (say) one hour, or one day