When no URL has been entered in the text editor field, the link target field is now hidden

Popups now have the option to change the target tab of a link.

Addressed Travis CI feedback

Addressed Travis CI feedback

Addressed GitHub feedback: colon removed from label, trackevent-editor changes removed, revision to text-editor

Removed whitespace

Revised popup-editor functionality
Merge branch 'bug947303' of https://github.com/kwkofler/popcorn.webmaker.org into bug947303

Conflicts:
	public/templates/assets/editors/popup/popup-editor.js
	public/templates/assets/editors/text/text-editor.js