Make MyPy happy

Fixes for various type errors throw by mypy.

portal/api/rest.py

 from flask import render_template
 
+from ..problem_details import ProblemResponse, not_found
 from .blueprint import bp
-from ..problem_details import not_found
 
 
 @bp.get("/<path:_>")
-def fallback_not_found(_):
+def fallback_not_found(_: str) -> ProblemResponse:
     """
     If you can't find a matching API endpoint, fallback to returning a 404.
 
@@ -16,5 +16,5 @@ def fallback_not_found(_):
 
 
 @bp.get("/")
-def index():
+def index() -> str:
     return render_template("redocly.html")