3859 [google_sign_in] fix registration links (cla: yes, p: google_sign_in)

3876 [google_sign_in] document web usage (cla: yes, p: google_sign_in)

3903 Enable web integration tests on stable (cla: yes, waiting for tree to go green, p: file_selector, p: google_sign_in, p: url_launcher, p: connectivity, platform-web)

3914 [google_sign_in] Migrate maven repo from jcenter to mavenCentral (cla: yes, waiting for tree to go green, p: google_sign_in, platform-android)