Use Devise for password resets

[gravitystorm]

I started working on using Devise, specifically for password resets. I figured that would be a straightforward place to start, since the functionality is straightforward, reasonably standalone, and it avoids some of our esoteric code around sessions.

However, after several hours, I didn't get very far. See https://github.com/gravitystorm/openstreetmap-website/tree/devise_passwords for WIP.

I need to pick apart some more of the existing code and align it further with devise, e.g. on new_user_password_path etc. There's also all of our stuff about case-insensitive email addresses, and I'm not sure how that will fit in with devise either.