Make MyPy happy

Fixes for various type errors throw by mypy.

portal/main.py

-from flask import Blueprint, render_template, redirect, url_for
+from flask import Blueprint, redirect, render_template, url_for
+from werkzeug import Response
 
 bp = Blueprint("main", __name__)
 
 
 @bp.get("/", defaults={"_": ""})
 @bp.get("/<path:_>")
-def index(_):
+def index(_: str) -> str:
     """
     Serve the index page at /*, i.e. at any path below / and at / itself.
 
@@ -16,5 +17,5 @@ def index(_):
 
 
 @bp.get("/api/")
-def redirect_to_api():
+def redirect_to_api() -> Response:
     return redirect(url_for("api.index"))