Renamed gh-search-input-trigger -> gh-power-select/input-trigger

refs https://github.com/TryGhost/Team/issues/973

- we have more uses for an input-based trigger for a power select component so the rename prepares the input-based trigger we use for the site search for more general usage
- removed hardcoded search icon and class, they can be passed in as options via the `@extra` param

ghost/admin/app/components/gh-power-select/input-trigger.hbs

@@ -1,5 +1,7 @@
 <div class="ember-power-select-search" {{on "mousedown" this.captureMousedown}}>
-    {{svg-jar "search" class="gh-nav-search-icon"}}
+    {{#if @extra.inputIcon}}
+        {{svg-jar @extra.inputIcon class=@extra.inputIconClass}}
+    {{/if}}
     <input type="search"
         {{autofocus}}
         {{did-insert this.registerInput}}

ghost/admin/app/components/gh-search-input.hbs

@@ -3,9 +3,13 @@
     @onChange={{action "openSelected"}}
     @placeholder="Search site..."
     @searchEnabled={{false}}
-    @triggerComponent="gh-search-input-trigger"
+    @triggerComponent="gh-power-select/input-trigger"
     @renderInPlace={{true}}
     @loadingMessage="Loading"
+    @extra={{hash
+        inputIcon="search"
+        inputIconClass="gh-nav-search-icon"
+    }}
     as |name select|
 >
     {{highlighted-text name.title select.searchText}}