Fix context menus

@njx 

This is a temporary workaround until marijnh/CodeMirror#1230 is merged.

The CodeMirror logic for handling context menus changed recently, which broke context menus on Chrome. 

**NOTE: This pull request is going into a new brackets-sprint20 branch, which diverges from master. This is only done to keep our fork stable. When updating the SHA, make sure you have the brackets-sprint20 branch checked out.**