Add bundlechange App that takes care of customers upgrades and downgrades

Things to consider:

• This commit adds a new Model, so migration is needed.
• 3 new configuration options are needed, if not added ldh_middleware will crash.
• When upgrading/downgrading from/to 'Family pack' forced_update does not take care of the invitees accounts. Upgrading from/to Family pack can be disable through the YAML file pointed by WOO_BUNDLES_INFO_FILE.

Changes:

limitmonitor.common module:

• Add billing_period to parse_subscription
• Add order_paid, cancel_subscription, get_user_external_credit, get_available_bundles and get_user_upgrade_choices

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

3 new config options:

• WOO_PARENT_PROD_ID
• WOO_UPGRADE_PAY_URL
• WOO_BUNDLES_INFO_FILE