Remove Flow type annotation infrastructure and lint TypeScript files

[LordSputnik]

Problem

The repository has numerous referenced to Flow type annotations, which now aren't used in the codebase. It also runs a flow linting plugin, which causes errors during the build process due to a hotfix to an earlier PR. Additionally, the "lint" build task doesn't run any linting on TypeScript files.

Solution

• Remove the babel and eslint flow plugins
• Remove the flow type definitions directory (flow-typed)
• Add the standard typescript-eslint plugin and migrate the rules which have slightly different definitions/checks in TypeScript
• Update the eslint parser from the babel-eslint parser to the typescript-eslint parser
• Update CONTRIBUTING.md and NODEJS_SETUP.md to reference TypeScript rather than Flow

Areas of Impact

No functional impact on codebase.

[MonkeyDo]

LGTM, thanks for popping this out so quick :)

[coveralls]

Coverage decreased (-0.006%) to 60.847% when pulling 4e0ca49 on LordSputnik:typescript into 091a7de on bookbrainz:master.