SessionInitiator URL construction should be more customizable

Issue Summary

Shibboleth SessionInitiator URL is built up as the following:

_HandlerURL_/_Location_

where
- _Location_ is the unique identifier of the SessionInitiator in /etc/shibboleth/shibboleth.xml
- _HandlerURL_ is the unique handlerUrl of the Application in /etc/shibboleth/shibboleth.xml, which can be an
-- absolute path (like https://my.site.com/Shibboleth.sso)
-- relative path (like /Shibboleth.sso, that should be the default), then HandlerURL should be _protocol_://HTTP_SERVER/_relative path_
- _protocol_ can be http or https (should default to https, customizable only if HandlerURL is a relative path)

"Login with Shibboleth" button should link this URL.

So the changes I'm proposing is:

• Add a configuration item handlerUrl with a default value of '/Shibboleth.sso'
• Add a configuration item of handlerProtocol only effective when the value of handlerUrl is a relative path
• (existing) WAYF location configuration entry should not contain '/Shibboleth.sso'