WIP: Add bundlechange App that takes care of customers upgrades and downgrades.

Changes included in this commit:

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

I have tested this in my test environment and is working but it will need thorough test in sandbox before going to production.

Things to consider:

• This commit adds a new Model, so migration is needed.
• 2 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.

Giving it a WIP title in case we want to wait and add forced_update code for taking care of 'Family pack'.