Change language from "Sign in with $AUTH_PROVIDER" to "Continue with $AUTH_PROVIDER"

Created by: felixfbecker

Part of RFC 213 Product issue: #12695 (closed)

To solve the underlying confusion around whether a Sourcegraph account is needed before someone can sign in with GitHub, change wording to “Continue with GitHub,” and include the option to “Continue with GitHub” on the sign up page (when configuration permits).