Add bundlechange App that takes care of customers upgrades and

downgrades.

Changes:

limitmonitor.common module:
 * Add billing_period to parse_subscription
 * Add get_variation, order_paid and cancel_subscription

limitmonitor.task_resources.subscription:
 * Process pending_regs in reverse order
 * Add upgrade_user_subscription and process_bundle_changes

limitmonitor.views:
 * Add can_upgrade control

middleware.settings:
 * Add BundleChange app
 * Add new config variable for BundleChange

purist:
 * Rename DeleteAcccountForm for reuse
 * Add display names to AccountType
 * Move spinner animation into a single template in purist module for
   reuse

Signed-off-by: Birin Sanchez <birin.sanchez@puri.sm>