User authentication and tracking

Adds code to validate JWT header from Google Identity-Aware Proxy, and saves logged-in user to database tables on model change.

Secondary account lead

• Adds secondary account lead attribute to Client model
• Add notes column to Client model
• Adds new model fields to Client list view and form.
• Also adds alembic library to project for managing database migrations

Initial release with complete filter and label settings

• Client view filters:
  • Active/Inactive Client
  • Account Lead
  • DFP Network ID
  • Product
• Link to Client view in main nav menu applies Active = Yes filter by default
• "Delete" control on Client list sets Client to Inactive, rather than removing from database (aka "soft" delete).
• Column headings and filter labels match business view rather than backend database column names
• Fixes bug in editable ajax form for manager in Employee list view