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/main.py

@@ -18,6 +18,8 @@ from quart import (
     flash,
 )
 
+import werkzeug.exceptions
+
 import babel
 import wtforms
 
@@ -48,7 +50,7 @@ class LoginForm(BaseForm):
 
 
 @bp.route("/-")
-async def index() -> quart.Response:
+async def index() -> werkzeug.Response:
     return redirect(url_for("index"))
 
 
@@ -56,7 +58,7 @@ ERR_CREDENTIALS_INVALID = _l("Invalid username or password.")
 
 
 @bp.route("/login", methods=["GET", "POST"])
-async def login() -> typing.Union[str, quart.Response]:
+async def login() -> typing.Union[str, werkzeug.Response]:
     if client.has_session and (await client.test_session()):
         return redirect(url_for('user.index'))
 
@@ -76,7 +78,7 @@ async def login() -> typing.Union[str, quart.Response]:
             password = form.password.data
             try:
                 await client.login(jid, password)
-            except quart.exceptions.Unauthorized:
+            except werkzeug.exceptions.Unauthorized:
                 form.password.errors.append(ERR_CREDENTIALS_INVALID)
             else:
                 await flash(
@@ -95,14 +97,13 @@ async def about() -> str:
 
     if current_app.debug or client.is_admin_session:
         version = _version.version
-        extra_versions["Quart"] = quart.__version__
         extra_versions["aiohttp"] = aiohttp.__version__
         extra_versions["babel"] = babel.__version__
         extra_versions["wtforms"] = wtforms.__version__
         extra_versions["flask-wtf"] = flask_wtf.__version__
         try:
             extra_versions["Prosody"] = await client.get_server_version()
-        except quart.exceptions.Unauthorized:
+        except werkzeug.exceptions.Unauthorized:
             extra_versions["Prosody"] = "unknown"
 
     return await render_template(