Login: Identify which ID's to add in Umbraco Core to ease finding some elements

[PeteDuncanson]

Suggest we try to get a PR in with a bunch of ids added to elements we want to easily get to. The templates in Umbraco Core were never written with testing in mind just "to get the job done" so adding some hooks in that we can latch onto would be super handy. Should be a quick win.

Only issue I can see is that we might get some name clashes if not careful. Might be worth name spacing the ids as a result, something like "login_forgottenEmailLink". Or something like that.

Elements I'm struggling with so far include the error messages (there are three of them), forgotten password form and link that shows it.