Fixed #14895 and #14919 - set SAML baseurl to a sensible default for docker users and users behind a reverse proxy

[uberbrady]

For users who are running Snipe-IT behind a reverse-proxy, the SAML implementation guesses an incorrect baseurl (as http:// rather than https://). This change sets it to a default based on the APP_URL - which we rely on elsewhere, as well as here within the SAML code itself. So this should be pretty safe.

[what-the-diff]

PR Summary

• Enhancement to the 'loadSettings' method
  The 'loadSettings' function in the 'Saml.php' file now includes a new key called 'baseurl'. The value of this key is derived from our app's URL combined with '/saml', improving the method's ability to dynamically determine the appropriate base URL for SAML functionalities.

[uberbrady]

Unfortunately, this is hard to test since this particular setup is pretty complex to reproduce :/