Removing login_form hook, plugins can now use template_construct hook and add an...

Removing login_form hook, plugins can now use template_construct hook and add an array index called login_form to the return value for that hook

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@11730 7612ce4b-ef26-0410-bec9-ea0150e637f0