add S3 prefix to settings
xml_url_prefix in Lab model, also change some default values
functions to get engine_xml, engine_file, and quiz_block_file urls (with prefix)
property to get engine_xml, engine_file, and quiz_block_file from lab instance
display engine_xml, engine_file, and quiz_block_file properly (with link),
custom form to validate engine_xml and quiz_block_file.
use base url from settings
use lab's property for engine file, engine xml, and quiz block file.
deactivate all before updating
support multiple correct answers