Customize return URL query form

[dvoytenko]

Closes #1574.

Few words why UrlReplacements is not used here directly: this code must be sync and it has to be part of the login-dialog.js because other methods of login (e.g. via viewer) will use different ways of assigning this value and thus it should propagate as RETURN_URL in the URL directly.

[dvoytenko]

@erwinmombay PTAL

[erwinmombay]

@dvoytenko can the user change the ret url to be something else besides the current url?

[dvoytenko]

@erwinmombay User can't change what return URL is - it's AMP that creates it and gives it to publishers to redirect to once they finish their flow.

[erwinmombay]

sorry, didn't mean user, meant "can the publisher customize the return url to be something else besides the current page url".

[dvoytenko]

Nope. I meant publisher as well. They actually can't customize any of the vars - we provide all of them :)

[dvoytenko]

Basically, a publisher says, I want "DOCUMENT_REFERRER" and we inject it. The same with "RETURN_URL", except that if they don't request it, we provide it anyway. You can think of "return" query parameter as a default for this. They can customize the query parameter name, but it will be sent in either case.

[erwinmombay]

ok, was just thinking if they would need that since redirecting to a different return_url is pretty common in normal pages. that wouldnt be applicable for amp pages?

[dvoytenko]

They can generally redirect anyway. But we explicitly ask to return to the URL we ask. This is essentially a way for them to return control to us. The ultimate goal, however, is to return back to the document that started the login flow.

[erwinmombay]

@dvoytenko thanks for explaining LGTM