Bump quart to version 0.17

This is needed because jinja2 had an update which caused the portal to
not work at all:

```
ImportError: cannot import name 'escape' from 'jinja2'
```

Quart needed updating for that.

This update required a lot of typefixes. Apparently, the "canned"
responses (like redirect) are now plain werkzeug responses, while
quart.Response does not inherit from werkzeug.Response (otherwise, we
could've changed the type annotations to werkzeug.Response everywhere,
but that doesn't work because a quart.Response is not a
werkzeug.Response).

P.S.: This time, I *did* check that avatar uploads don't break (see
b007afc).

snikket_web/__init__.py

@@ -18,6 +18,8 @@ from quart import (
     jsonify,
 )
 
+import werkzeug.exceptions
+
 import environ
 
 from . import colour, infra
@@ -40,7 +42,7 @@ async def proc() -> typing.Dict[str, typing.Any]:
 
     try:
         user_info = await infra.client.get_user_info()
-    except (aiohttp.ClientError, quart.exceptions.HTTPException):
+    except (aiohttp.ClientError, werkzeug.exceptions.HTTPException):
         user_info = {}
 
     return {
@@ -105,16 +107,16 @@ async def backend_error_handler(exc: Exception) -> quart.Response:
 
 
 async def generic_http_error(
-        exc: quart.exceptions.HTTPException,
+        exc: werkzeug.exceptions.HTTPException,
         ) -> quart.Response:
     return quart.Response(
         await render_template(
             "generic_http_error.html",
-            status=exc.status_code,
+            status=exc.code,
             description=exc.description,
             name=exc.name,
         ),
-        status=exc.status_code,
+        status=exc.code,
     )
 
 
@@ -200,19 +202,19 @@ def create_app() -> quart.Quart:
     app.context_processor(proc)
     app.register_error_handler(
         aiohttp.ClientConnectorError,
-        backend_error_handler,  # type:ignore
+        backend_error_handler,
     )
     app.register_error_handler(
-        quart.exceptions.HTTPException,
+        werkzeug.exceptions.HTTPException,
         generic_http_error,  # type:ignore
     )
     app.register_error_handler(
         Exception,
-        generic_error_handler,  # type:ignore
+        generic_error_handler,
     )
 
     @app.route("/")
-    async def index() -> quart.Response:
+    async def index() -> werkzeug.Response:
         if infra.client.has_session:
             return redirect(url_for('user.index'))