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.html

@@ -1 +1,9 @@
-<app-dashboard></app-dashboard>
+<div class="header" fxLayout="row" fxLayoutAlign="space-between center">
+    <div fxLayout="row" fxLayoutGap="16px" fxLayoutAlign="start center">
+        <img src="assets/logo-horizontal.svg" width="150px" />
+        <span>Tech UI</span>
+    </div>
+    <button mat-stroked-button (click)="restoreDefaultTheme()">Restore default theme</button>
+</div>
+
+<app-dashboard></app-dashboard>