Add SSO login support

- Add [sso] config section with redirect_uri
- Create mcdsl/sso client when SSO is configured
- Add /login (landing page), /sso/redirect, /sso/callback routes
- Add /logout route
- Update login template with SSO landing page variant
- Bump mcdsl to v1.6.0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/spf13/afero/appveyor.yml

@@ -0,0 +1,10 @@
+# This currently does nothing. We have moved to GitHub action, but this is kept
+# until spf13 has disabled this project in AppVeyor.
+version: '{build}'
+clone_folder: C:\gopath\src\github.com\spf13\afero
+environment:
+  GOPATH: C:\gopath
+build_script:
+- cmd: >-
+    go version
+