Revert "Replace image-based AutoLogin with iframes"

This reverts commit 4d4600900ed709b5fe6ffb3351372b55014e0584

(Except the new i18n messages were kept, so we don't make more work
for the translators.)

Change-Id: I3a03e4539bae4da7f58fddfb189cc01b5ee737d8