Port Datasette from Sanic to ASGI + Uvicorn (#518)

Datasette now uses ASGI internally, and no longer depends on Sanic.

It now uses Uvicorn as the underlying HTTP server.

This was thirteen months in the making... for full details see the issue:

https://github.com/simonw/datasette/issues/272

And for a full sequence of commits plus commentary, see the pull request:

https://github.com/simonw/datasette/pull/518

datasette/views/index.py

@@ -1,9 +1,8 @@
 import hashlib
 import json
 
-from sanic import response
-
 from datasette.utils import CustomJSONEncoder
+from datasette.utils.asgi import Response
 from datasette.version import __version__
 
 from .base import BaseView
@@ -104,9 +103,9 @@ class IndexView(BaseView):
             headers = {}
             if self.ds.cors:
                 headers["Access-Control-Allow-Origin"] = "*"
-            return response.HTTPResponse(
+            return Response(
                 json.dumps({db["name"]: db for db in databases}, cls=CustomJSONEncoder),
-                content_type="application/json",
+                content_type="application/json; charset=utf-8",
                 headers=headers,
             )
         else: