Don't kill SAML-session on logout

[mrsimpson]

Description:

When using SAML-session, ligging out should end the session of the chat, but not (necessarily) the SAML session. Other applications shall still be able to use the session.

Server Setup Information:

• Version of Rocket.Chat Server: 0.59.3

Steps to Reproduce:

1.Configure a SAML provider
2. Log.in using SAML to another application
3. Log-in using SAML to Rocket.Chat
4. Log-out from Rocket.Chat
5. Open the other application again

Expected behavior:

Other applications using the same SAML-session should still be authenticated

Actual behavior:

The SAML-session is killed

Proposal

Allow the admin to configure per SAML-provider how the logout using the provider shall behave:

• end the SAML session (default for compatibility)
• only end the RC-session