Add an option to directly send the URL without trampolines

[shhyou]

This PR adds an option in the browser preference to send URLs directly without going through trampolines. Comments welcome!

• [x] Draw the label of the checkbox from string-constants
• [ ] docs
• [x] Code cleanup

[rfindler]

This looks okay to me, except the string constants issue and the docs. There is a document that has all of the checkboxes and menu items in DrRacket with some explanation of them here. Maybe adding a note there with a index entry or something like "documentation on chromebooks" or similar might work?

[shhyou]

Thanks! I forgot about the docs.