Add 2 new hooks to fix mobile logins

Bug 51789 is caused by loginwiki always redirecting back to desktop domain -
this patch gives MobileFrontend a chance to save the mobile host and then
redirect to it.

Corresponding MobileFrontend change: https://gerrit.wikimedia.org/r/75155

Change-Id: I3991ae4d90b3943e75cb2b869074094e56f2ccbf