Add OAuth2 logout support

[rogerdahl]

The Logout function in the Portal clears the logged in session, but this does not change the logged in status at the IdP. We currently handle this by requiring a login prompt for each login, but we should also incorporate an actual OAuth2 logout flow for the IdPs that support it (it's a fairly new addition to the OAuth2 standard). An example of this is the Logout function on MyUNM, which uses Microsoft as the IdP. After logging out, the final page is https://login.microsoftonline.com/common/oauth2/logoutsession, with custom UNM styling.