Add phx.gen.auth generator

[aaronrenner]

This adds the mix phx.gen.auth task from phx_gen_auth v0.6.0 into phoenix master. The only additional change is to address a test failure caused by some changes to current_path/1 on phoenix master (see aaronrenner/phx_gen_auth#96).

This PR is missing the phx.gen.auth overview doc (https://github.com/aaronrenner/phx_gen_auth/blob/master/guides/overview.md). Is that something you'd like as part of this PR and if so, where would be a good place to work this in? We'll also probably want to add this to phoenix's mix tasks guide in a future PR.

Let me if you see anything stylistically that needs to be updated to match the rest of the phoenix codebase or anything else that would be good to change. Thanks!

/cc @josevalim

[josevalim]

Should this be moved inside the describe block above?

[josevalim]

The same applies to the test files, btw. :)

[aaronrenner]

Thanks for catching that. I think I fixed them all in 02e8988.

[josevalim]

@aaronrenner just some minor comments but otherwise it looks great to me!

Regarding the guides, we need to do three changes which can be done separately:

1. Bring in the guide under a new subsection called "Authentication", that comes right under subsection "Guides". The current guide can now be called "mix phx.gen.auth" and I believe it should be a mixture of the current README and the guide. I.e. it should explain how to use the generator and also describe the security considerations.

2. Update the mix task guide to mention mix phx.gen.auth and simply link to the guide above

3. We should probably update the context guides to use another example beyond authentication /cc @chrismccord

EDIT: Once we do step 1, let's also add a link to the guides from the generator.

[josevalim]

LGTM! Feel free to merge it at your discretion, if you would rather add the guides now or later, it is your call. ❤️