Set a callback function to all forms

Added a generic callbackfunction (always returns true) to set to "only-a-
button-forms".

Bug: 68097
Change-Id: I52b819068af21f71d1752591cd30a5d58dd318e4
Make keep login configurable

New variable $wgGLShowKeepLogin handles, if the keep login checkbox for
GoogleLogin on user login page is visible or not. Default to "true" (visible).

Change-Id: I080131e6ab66beecee3d0a9dbb7f93eb5ce9438d
Merge "Set a callback function to all forms"