Fixed external storage status indicator in admin page

- Fixes #5241
- Fixed indicator to appear, its selector was wrong
- Added spinner while saving an entry's settings
- Removed ajax "async: false" that blocked the browser while saving
Now using smaller spinner image for status indicator

To make the status indicator appear smaller, even in IE8 that doesn't
support background-size, there is no a smaller animated gif
"loading-small" for that purpose.
Merge pull request #5253 from owncloud/externalstorage-adminstatusindicator

Fixed external storage status indicator in admin page