next-gen-gui: Add button to restore default theme (#7433)

This adds a button in the top right that changes the config back to the
default theme.

Code wise, it takes the header that was previously a part of the
dashboard component and moves it to the app component, and then adds the
button there. Possibly the header should be a component of it's own, but
that's more of refactor that can happen separately I think.

The config change uses the new config API to just patch the relevant
setting.

I'm not doing an automatic reload because 1) I don't want to figure out
how to do it correctly and 2) this doesn't work reliably anyway, as
for example the current gen GUI does a reload and ends up with
connection refused as the API service is still reloading...

next-gen-gui/src/app/app.component.ts

@@ -1,4 +1,6 @@
 import { Component } from '@angular/core';
+import { SystemConfigService } from './services/system-config.service';
+import { MessageService } from './services/message.service';
 
 @Component({
   selector: 'app-root',
@@ -6,5 +8,16 @@ import { Component } from '@angular/core';
   styleUrls: ['./app.component.scss']
 })
 export class AppComponent {
-  constructor() { }
+  constructor(
+    private systemConfigService: SystemConfigService,
+    private messageService: MessageService,
+  ) { }
+
+  restoreDefaultTheme(): void {
+    this.systemConfigService
+      .setGUITheme('default')
+      .subscribe(() => {
+        this.messageService.add('The default GUI theme has been selected. Please hit "Reload" in your browser.')
+      })
+  }
 }