Add link to download public certificate for SPs to validate SAML responses

Changes:
* Add certificate file (public key) in Stub App_Data.
* Add a link in index to download it.

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Hello there devs!
I wanted to start slow, with this simple issue since I am still learning the basics of git.
Please provide all critics and reviews possible, or even discard this request if the solution is a mess.
Thank you.