Drag-n-drop directive

Add aw-drop-file directive to a textarea form element. User can drop a file (any file) onto the field and the contents as text will be placed in the textarea. Added this directive to credentials ssh_key field.

awx/ui/static/js/forms/Credentials.js

@@ -240,6 +240,7 @@ angular.module('CredentialFormDefinition', [])
                 },
                 addRequired: false,
                 editRequired: false,
+                awDropFile: true,
                 'class': 'ssh-key-field',
                 rows: 10
             },