chore: added base.html file to assignments app, modified index and details templates to extend base.html file

fix: moved images and style.css to new folder, updated corresponding dir in Assignment model and index and details templates

feat: modified passing_score to be able to automatically calculate 60% of max_score in Assignment model

chore: added another record to the Assignment model to demonstrate the many-to-one relationship between Assignment and Course

chore: edited fields of Assignment and Course models to better reflect the subjects taught by the widget users

chore: populated the Assignment model with a third record using Django admin, forgot to commit for Lab 2