chore(workshops): ignore the dead sign_up_url column

[mroderick]

Part of #3018. Sets up the drop of the dead workshops.sign_up_url column, following the two-step approach proven in the invites-sent and unsubscribed stacks.

sign_up_url is a nullable string whose only purpose (an external RSVP link in workshop partials) disappeared when workshop pages moved to invitation-based RSVP in c9391a35 (2014-09-09). Nothing has read or written it since; the production snapshot has 3 non-null values of 2603 rows, all set before 2014. This PR adds the column to self.ignored_columns on Workshop so Ar stops selecting it and models cannot reach the attribute. The second, stacked PR removes the column.

Detail

sign_up_url appears nowhere outside db/schema.rb. The legacy-column spec in spec/models/workshop_spec.rb is transient: the stacked drop PR removes it together with the attribute watcher role it plays (same pattern as meetings).

Review notes

• Nothing risky: mirrors the merged chore(meetings): ignore PR one column over.
• Deliberately not done: the column removal itself — that is the stacked PR, which merges after this one deploys and soaks.

[mroderick]

This has been deployed as part of v794