Add a relaxed-idna feature to allow some uses of _ in hostnames.

This is added to aid migration from Codeberg Pages v2. Forgejo allows
both `_` and `-` in usernames, and it is necessary to be able to accept
host names like `user_name.codeberg.page` under a wildcard domain.
(It is not possible to get a TLS certificate for a host name like this,
so only a wildcard certificate will be able to cover it.)

.forgejo/workflows/ci.yaml

@@ -28,6 +28,9 @@ jobs:
       - name: Build service
         run: |
           go build
+      - name: Run tests
+        run: |
+          go test ./src
       - name: Run static analysis
         run: |
           go vet