Add eslint and "lint" npm scripts

[eheikes]

Adds linting to the project (for the client and server packages) using ESLint and lint npm scripts. It does both traditional linting (looking for problematic patterns) and style checking (enforcement of conventions). The rules were selected to match the existing code as closely as possible.

Run lerna run lint to test it out.

[djhi]

Thanks but I'd rather have prettier handle the formatting itself by adding eslint-prettier-config and eslint-prettier-plugin

[eheikes]

Okay, I added the Prettier plugins and use them to format the code.