New method: get logged in user's email address

[SiAdcock]

We should expose a method that returns the user's email address iIf a user is signed in. This will allow us to pre-populate forms in DCAR e.g. the newsletter signup form

[michaelclapham]

I've got this draft PR to add this method to Bridget here #259
Could someone review this change? Once this change is merged I can update my draft
Android and iOS PRs that implement the new method, and someone on newsletters dev team should be good to use the call the new method from the web layer.
FYI: @shesah