Support fragments in redirect destinations

Problem/Motivation

Fragments do not seem to be supported in the redirection feature.

Steps to reproduce

Set a simplesamlphp_auth_returnto cookie with value https://example.com/some-path?some-param#some-fragment and sign in.

Note that the URI that's landed on is https://example.com/some-path?some-param, sans fragment.

Proposed resolution

Respect the fragment.

Remaining tasks

User interface changes

API changes

Data model changes