task/ta-144 -- Identify table fields to update

top/adp.py
* new method sanitise_agent_status() that prepares the agent.status
  value for further table column adjustment
* new method filter_update_fields() that remove keys from a given
  dictionary if the value is not an empty string or not None
* new method sanitise_agent_parcel_size_code() that prepares the
  agent.parcel_size_code value for further table column adjustment
* new method sanitise_delivery_partner_id() that converts the string
  value to the equivalent delivery_partner.id
* new method sanitise_login_account_status() that prepares the
  login_account.status value for further table column adjustment
* refactor the sanitise() method to utilise the new
  sanitise_agent_status(), sanitise_agent_parcel_size_code(),
  sanitise_login_account_status and sanitise_delivery_partner_id()
  methods