enable ability to use rxjs-compat

**I'm submitting a ...** (check one with "x")
```
[X] bug report => search github for a similar issue or PR before submitting

**Current behavior**
Existing project wanting to update project to current state of seed. Problem is have lots of code to update for the rxjs breaking change. so wanted to still update but have time to update the code as i could.

**Expected behavior**
existing code to compile and app to execute as expected

**Minimal reproduction of the problem with instructions**
rxjs-compat isn't established in the seed anywhere so that it doesn't break existing code.

**What is the motivation / use case for changing the behavior?**
ability to upgrade existing projects to Angular 6 and rxjs 6 and update code as can to fix the breaking change

* **Node:** `node --version` = 8.11.2

Start by inspecting the seed's dependency and build configuration to see how rxjs-compat could be established, then run the existing project with Node 8.11.2. Done means existing Angular 6 and RxJS 6 projects compile and execute without immediately requiring all RxJS breaking-change updates.

Written by the indexing model from the issue text.