feat: Add ApiService

[chloe463]

WHY & WHAT

• This is an angular tutorial for [Public] Frontend Night #23 wantedly/frontend-night#23
• Add ApiService using HttpClientModule in @angular/common/http

BASICS

1. Install angular-in-memory-web-api
2. Import HttpClientModule and HttpClientInMemoryWebApiModule from libraries to AppModule
3. Create mock database. (Just a ts file)
4. Generate ApiService using @angular/cli command (ng g s services/api)
5. Add functions to ApiService and refactor AppComponent to make it possible to call (mock-) WebAPI.

ADVANCED

6. Add switchMapTo to avoid nested subscribes
7. Use async pipe in angular-template.