Fixed tabbing in and out of <GhInputWithSelect>

refs https://github.com/TryGhost/Team/issues/1009

- setting `tabIndex="-1"` on the trigger prevents the trigger container receiving focus rather than the container input element (fixes general tabbing in and out)
- added extra blur handling to the input element so that it is not left in an open dropdown state when tabbing out

ghost/admin/app/components/gh-input-with-select/index.hbs

@@ -53,7 +53,7 @@
         @searchPlaceholder={{@searchPlaceholder}}
         @selected={{@selected}}
         @selectedItemComponent={{@selectedItemComponent}}
-        @tabindex={{@tabindex}}
+        @tabindex="-1"
         @triggerClass="ember-power-select-multiple-trigger {{@triggerClass}}"
         @triggerComponent={{component (or @triggerComponent "gh-input-with-select/trigger") tabindex=@tabindex}}
         @triggerId={{@triggerId}}

ghost/admin/app/components/gh-input-with-select/trigger.js

@@ -58,7 +58,17 @@ export default class GhSearchInputTrigger extends Component {
     }
 
     @action
-    handleBlur() {
+    handleBlur(event) {
+        if (event?.relatedTarget) {
+            const thisInputTrigger = this.inputElem.closest('.ember-basic-dropdown-trigger');
+            const relatedInputTrigger = event.relatedTarget.closest('.ember-basic-dropdown-trigger');
+
+            if (relatedInputTrigger !== thisInputTrigger) {
+                this.args.select.actions.search('');
+                this.close();
+            }
+        }
+
         if (this.args.extra.value && this.args.select.searchText === this.args.extra.value) {
             this.args.select.actions.search('');
             this.close();