Minor updates to the configProviders.js

* Check if url for newznab provider being added starts with http|https, if not just add http://.
* Abort adding provider if url is empty.
* Trim leading and trailing spaces from the url for provider before any logic is ran on it.
* Correct extension of default newznab provider image from .gif to .png.
* Added some missing semicolons and cleaned up some trailing spaces in the js file.