Add pre_loginpage_hooks

[Daniel Thee Roperto]

Give a chance for auth plugins to authenticate the user or perform other tasks before showing the login page.

This is similar to https://tracker.moodle.org/browse/MDL-48887

[Daniel Thee Roperto (roperto)]

I am adding a patch,

Please let me know if you prefer a pull request.

[Kristina Hoeppner (kris-hoeppner)]

Hi Daniel. It would be great if you put it into our review system Gerrit as per https://wiki.mahara.org/wiki/Developer_Area/Contributing_Code

Thanks, Kristina

[Mahara Bot (dev-mahara)]

Patch for "master" branch: https://reviews.mahara.org/9098

[Mahara Bot (dev-mahara)]

Reviewed: https://reviews.mahara.org/9098
Committed: https://git.mahara.org/mahara/mahara/commit/bfdba3375f3f763d654b2413895b86aa744f76cf
Submitter: Robert Lyon (<email address hidden>)
Branch: master

commit bfdba3375f3f763d654b2413895b86aa744f76cf
Author: Daniel Thee Roperto <email address hidden>
Date: Thu Aug 30 16:26:29 2018 +1200

Bug 1749339: Add pre_loginpage_hooks

Add a pre-login hook to check if user is already logged in,
giving each auth plugin the chance to authenticate or redirect the user as
needed.

behatnotneeded

Change-Id: I690fd174a02e9ffd26b8e3c8f3efad041b54d3da