Per-Campaign Mandrill Template Selection

Description:
Currently, all Pelcro campaigns (promotional, renewal, cancellation) share a single Mandrill template (campaign_triggered). Publishers who want to run multiple campaigns with distinct email designs — e.g. a monthly renewal reminder and a seasonal promotional offer — must manually edit the template in Mandrill's UI each time they switch campaigns. This creates operational overhead and is error-prone.

Requested Behaviour:
Allow each campaign to specify its own Mandrill template. When a campaign runs, the email send uses that campaign's assigned template rather than the global campaign_triggered fallback.

Use Cases:

• A recurring monthly renewal campaign uses a "renew your subscription" template automatically, every month, without manual intervention

• Ad-hoc promotional campaigns (Back to School, end-of-year offers, etc.) are configured once with their own template and target segment, then triggered on a scheduled date — no Mandrill UI changes required

• Multiple campaigns can coexist and run independently with different messaging and designs

Expected Value:
Publishers can fully configure a campaign once and let it run on schedule. Removes the need to touch Mandrill's interface between campaign runs, enabling true campaign automation.

Scope Notes (for engineering):

• Add a mandrill_template field to the Campaign model

• Pass the campaign's template through the Campaign\Triggered event to the Mandrill listener, overriding the global campaign_triggered lookup

• Validate the template exists and is published in the account's Mandrill instance on save (same validation pattern already used for transactional templates)

• UI to select from available templates (fetched from Mandrill API) when creating or editing a campaign