add login component and auth service

admin/src/app/components/login/login.component.html

@@ -0,0 +1,15 @@
+<div>
+  <h2>Login</h2>
+  <form [formGroup]="loginForm" (ngSubmit)="onSubmit()">
+    <div>
+      <label for="username">Username:</label>
+      <input id="username" formControlName="username" />
+    </div>
+    <div>
+      <label for="password">Password:</label>
+      <input id="password" type="password" formControlName="password" />
+    </div>
+    <button type="submit" [disabled]="loginForm.invalid">Log In</button>
+  </form>
+  <p *ngIf="errorMessage">{{ errorMessage }}</p>
+</div>