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/dustin/go-humanize/.travis.yml

@@ -0,0 +1,21 @@
+sudo: false
+language: go
+go_import_path: github.com/dustin/go-humanize
+go:
+  - 1.13.x
+  - 1.14.x
+  - 1.15.x
+  - 1.16.x
+  - stable
+  - master
+matrix:
+  allow_failures:
+    - go: master
+  fast_finish: true
+install:
+  - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
+script:
+  - diff -u <(echo -n) <(gofmt -d -s .)
+  - go vet .
+  - go install -v -race ./...
+  - go test -v -race ./...