Back TABLE_A_EMPLOYER_NIC with static TABLE_A_EMPLOYER_NIC_BRACKETS, allowing this to be used for lookup of thresholds

This makes a TABLE_A_EMPLOYER_NIC_BRACKETS data structure constant, with this then used to populate the existing TABLE_A_EMPLOYER_NIC dictionary of nic calculator functions. This way the underlying data can be queried to provide details of thresholds (eg. for uis/devops/research/staff-on-costs-calculator/webapp#15 (closed))