Feature: Import folders of questions from STACK question library

[EJMFarrow]

This could be done by updating stack_question_library::get_file_list() to add parent folder info to files and to add a flag to files to indicate if they have files as immediate children. This info could then be used to place buttons in the correct places in the file tree that import all the questions in the folder.

• Making the import recursive would be harder (and more dangerous). Would we create categories, for instance?
• All the questions in the folder would be imported even if some have been imported already. (We could prevent this for a given access of the library but is it worth it?)
• Do we loop over requests or send all the required question identifiers in one go? First option is simpler (particularly for error handling). However, Feature: Import quiz from STACK question library #1358 requires the load of a folder in a single request.
• How does this look to the user? Should they only be able to import a folder if they're viewing one of the questions? (If so, is in the directory tree the correct place for the button?) Do we need a progress bar (which would only be possible with looped requests)? How is success/failure displayed?