Subject line and first line variables fill in again

Fixes from August 17 to 23, most of them in campaign sending.

• Campaigns whose subject is {{Subject Line}}, or whose body uses {{First Line}} or {firstName}, failed to build their emails. Ken now fills them from each lead's AI lines and custom variables, including copy written per segment. A lead missing a required AI variable is held back instead of getting a blank.
• Follow-up steps with https links were rejected. They send now.
• Importing leads from another campaign or a segment dropped the campaign's custom variables, so they did not work in sequences or AI prompts. They now come across, up to 50 per campaign.
• AI custom fields saved under a display name, such as "Renewal Date", were left out of qualification, personalization, rewriting and segmentation prompts. Ken includes them now.
• A duplicated campaign kept tracking links that pointed at the original. The copy now gets its own links in subjects and bodies.
• A tag you set on a reply by hand could be replaced by Ken's AI classification. Your tag now stays.