Fixed location of OAuth link. Removed instance member github_username from CLI

[richwblake]

Current OAuth link points to a depreciated site. It now points to the link https://base.flatironschool.com/account/manage which is where the key is now located. I removed the instance member github_username from the CLI class, as its only use was to interpolate their specific username into the CLI output. This is no longer needed, as the new link redirects the student to their specific page. Additionally, I removed the variable github_username from the Setup class, which was used as an argument to CLI's initialize method. Because CLI's github_username member has been removed, the init call no longer requires arguments.