Fixes for Talk feature

* Use Overlay.hogan, instead of talk.hogan as base Overlay template

* Fix Bug 70989

* Fix missing options when add a new discussion to a non existing talk page
(Bug 71028)

* Remove error code from error message when post new discussion and add
more specific error messages for some errors

Bug: 70989
Bug: 71028
Change-Id: Ic9a4d6b216509d28e4fa24869372c41fe31eb7a6
Merge "Fixes for Talk feature"