Remove Private Tokens

To do:

• Remove Private API Token from profile/account
• Remove API::Session which can be used to get the private token when providing username/password
• Remove UserWithPrivateDetails entity.
• Remove all API/web authentication using private_token or authentication_token (they're the same thing)
• Sudo scope (lib/api/helpers.rb:432)
• Migrate Private tokens to PATs with the api scope, as well as the sudo scope if the user is an admin (depends on https://gitlab.com/gitlab-org/gitlab-ce/issues/38447)
• Remove users.authentication_token
• Update API docs (doc/api/README.md, doc/user/profile/personal_access_tokens.md)
• Move RSS and incoming email tokens to Access Tokens (app/views/profiles/accounts/show.html.haml)
• Fix specs
• Add new specs
• Add changelog item

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/38595 and https://gitlab.com/gitlab-org/gitlab-ce/issues/38447