FIX: make composer full screen shortcut work when inputs have focus (#6907)

- Uses a Mousetrap plugin for global shortcuts
- Implemented for search `ctrl+alt+f` and composer fullscreen `shift+f11` shortcuts

app/assets/javascripts/discourse/templates/composer.hbs

@@ -111,7 +111,8 @@
                             importQuote=(action "importQuote")
                             togglePreview=(action "togglePreview")
                             showToolbar=showToolbar
-                            afterRefresh=(action "afterRefresh")}}
+                            afterRefresh=(action "afterRefresh")
+                            focusTarget=focusTarget}}
 
           <div class='submit-panel'>
             {{plugin-outlet name="composer-fields-below" args=(hash model=model)}}