v5.1.0

• Added client shipping address fields (address_2,city_2,etc) accessible in templates
• Added client industry, size, id, and VAT fields
• Added client contact firstname, lastname, title, phone, fax, mobile, primary , optin and note fields
• Added individual Client payment terms and accessed by due_at (if not set defaults to invoice config value)
• Converted many events/listeners to model observers
• Expand vendor info
• General categories and vendors for expenses and products
• App name/namespace/language to BT
• remove FusionInvoice 2018-8 conversion from setup
• Added Purchase Order Module which connects to vendors and products
• Added config for collapsed side menu
• corrected invoice update when payment amount is changed.
• corrected payment emailing
• added product inventory tracking ability to sent invoices.
  note:previously unused "type" column of products table has been changed to "inventorytype_id"
  Any existing entries in the type column will be migrated to the inventory_types table and updated
  in the product. This table is currently not editable within the application.
  This is a breaking change if you have any custom code that is accessing the type column in the products table.
• Added Database clean/purge tool
• Revamped Documentation
• Update to Laravel 6.0
• Update deprecated Fixer.io API for currency exchange rates. To use currency exchange rates in BillingTrack,
  you have to signup for a (free) API key at https://fixer.io. Once you get the key,
  Enter it in System Settings - General - FixerIO API Key
• updated compatibility with latest Paypal, Mollie and Stripe API's
• Remove zero balance invoices from Overdue status filter