Add `eslint-plugin-import` and `eslint-plugin-prettier`

[cball]

We've been using eslint-plugin-import on its own, but we need to add to this plugin.

eslint-plugin-prettier is a dependency but not listed in package.json.

[cball]

We will now be using this on web and mobile. Here's the most recent config:

module.exports = {
  rules: {
    'import/order': [
      'warn',
      {
        groups: ['builtin', 'external', 'internal', 'parent', 'sibling', 'index'],
        'newlines-between': 'always',
      },
    ],
  },
};