lp2kanban will lose track of blueprint-linked cards if they get linked to a merge proposal

The following situation will cause lp2kanban to stop updating a blueprint's work item from a card:

 * Card linked to blueprint using external link field
 * Card linked to bug using external id field
 * Merge proposal linked to bug via branch (This causes the external_system_url field on the card to be updated with the MP URL)
 * After this point, bugs2cards.sync_board() will no longer treat the card as needing to be synced with a blueprint, since it uses board.getCardsWithExternalLinks() to find the cards it needs to sync.

Proposed solution:

 * Stop using board.getCardsWithExternalLinks(); instead, we should create a getCardsWithBlueprints() or some such that checks for card.description_annotations.blueprint_url.