introduce typescript to backend

[stevenhao]

This PR is 95% refactor (shouldn't affect any behavior) but also contains some scattered work in progress from https://github.com/downforacross/downforacross.com/pull/130 and some minor changes to db client connection management

major changes

• server is split into files & uses ts
• server modularization done via es6 modules instead of commonjs modules w/ OOP interfaces
• postgres client pooling, should improve performance
• tsconfigs for server, frontend
• shared directory to hold common code, tsconfig.base.json to workaround create-react-app (see https://github.com/facebook/create-react-app issue 5645)
• server/client communication endpoints are elevated to each live in isolated source files; request initiator & request handler code placed in src/api/:route_name and server/api/route_name respectively.
  • i might change my mind on this organization.
  • uses express router composition on backend + new request type parameterization from @types/express@4.17.2

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