Upgraded no-restricted-require in shared rule to error

- We should not require server or frontend files inside the shared libraries as these are intended to be required FROM the server and frontend components
- Now that we've resolved the two outstanding warnings, make this an error so we can't regress on this easily

.eslintrc.js

@@ -31,7 +31,7 @@ module.exports = {
         {
             files: 'core/shared/**',
             rules: {
-                'ghost/node/no-restricted-require': ['warn', [
+                'ghost/node/no-restricted-require': ['error', [
                     {
                         name: path.resolve(__dirname, 'core/server/**'),
                         message: 'Invalid require of core/server from core/shared.'