Remove deprecated context and print functions from auth/saml

The auth/saml module should now be fully ready for 2.7 support. Deperecated functions have been changed to their 2.7 equivalents. `get_context` changed to `CONTEXT::instance` and `print_continue` changed to `$OUTPUT->continue_button`.

I also fixed a notice about empty object created, when saml_config.php doesn't exist.

cc @henrikthorn