Improved external share dialog

OC.dialogs now returns the promise to make it possible for the called to
do additional operations after the promise is resolved.

Changed title, label and buttons.
Removed icon.

Please review @owncloud/designers @icewind1991 @schiesbn 

:warning: protect your eyes, hacking involved!

Feel free to push if you want to change the wording.
The best way to test it is to comment out the line with `location.href` in `external.js` to force the dialog appearing every time on refresh. (convenient for testing)

Edit: removed wrong screenshot