URI encode parts of the extension download URL

This fixes https://github.com/adobe/brackets/issues/8692.
The extension name and the extension version will now be URI encoded and all URL unsafe characters are replaced by their `%` encoded entities.
This is backwards... it should be `expect(formattedURL).toBe("http://...")` (`expect` takes the ["actual" value](http://jasmine.github.io/1.3/introduction.html#section-Expectations))