This commit moves the extensions project, formerly housed in a separate repo, into the ecommerce project as a new app. This change will accelerate development of the ecommerce service by consolidating changes in one repo, thereby avoiding the need for convoluted installation procedures and preventing the need to change code in two repos whenever extensions is modified.

Settings have been reorganized to reflect the new project layout. Since the ecommerce project authenticates users against the LMS instead of using Oscar's default login forms, Selenium tests of the management dashboard which formerly relied on the default login page have also been modified to use a new auto-auth endpoint.