This doesn't support setting accounts up for Google login: that still
has to be manually done via the database by setting the email field to
`username@gmail.com@`, where the second `@` denotes that it's using
Gmail login.
If the email field does end in `@`, `getassertion` will note this by
sending `;;@gmail`, to convey that the server is expecting a Google
login token rather than a password.
Upon receiving `;;@gmail`, the client will replace the password box will
with a Google login button, and then send the resulting Google login
token to the server in the `password` field. The server will validate
the "password" using the Google server libraries, and otherwise handle
the login as normal.
Note that Google login requires various features that a paranoid person
might disable; most notably 3rd-party cookies.
FixesZarel/Pokemon-Showdown#3394
