small regex fix to match entire remainder of filename that ends with _append
block invokation of File.open in file creation to prevent memory leak
import_csv.rb two small changes

* changed APPEND_FILE_MATCHER again and gsubs so as to exhibit intended behavior
* cached sorted_filenames
import_csv.rb three more small changes

* append directly to @messages in pertinent branch within process_file method rather than store to intermediate variable msg, to prevent possible error for referencing undefined variable
* renamed final import method csv_importer to import_csv to avoid confusion with csv_importers method
* restructured conditional branching within try_to_unzip method to be more readable
import_csv.rb changes

* ensured that files are not processed if there is a skyward_student error because of enrollments
* also deleted private method file_exists? which is no longer being called and refers to a model_name which no longer appears to be relevant