Fix award-emojis alert flash message

This adds a new feature to `Flash` that allows to pin it after a
specified selector. This removes a fixed position from such award-emoji
alert, and makes it responsive by design of selector that this alert is
pinned to.

Closes #3996

app/assets/javascripts/flash.js.coffee

@@ -12,5 +12,5 @@ class @Flash
     @flash.click -> $(@).fadeOut()
     @flash.show()
 
-  pin: ->
-    @flash.addClass('flash-pinned flash-raised')
+  pinTo: (selector) ->
+    @flash.detach().appendTo(selector)

app/assets/javascripts/notes.js.coffee

@@ -114,7 +114,7 @@ class @Notes
     unless note.valid
       if note.award
         flash = new Flash('You have already used this award emoji!', 'alert')
-        flash.pin()
+        flash.pinTo('.header-content')
       return
 
     # render note if it not present in loaded list

app/assets/stylesheets/framework/flash.scss

@@ -15,13 +15,3 @@
     @extend .alert-danger;
   }
 }
-
-.flash-pinned {
-  position: fixed;
-  top: 80px;
-  width: 80%;
-}
-
-.flash-raised {
-  z-index: 1000;
-}