Mechanism for creating custom pages using templates

Closes #648

datasette/app.py

@@ -17,6 +17,7 @@ from markupsafe import Markup
 import jinja2
 from jinja2 import ChoiceLoader, Environment, FileSystemLoader, PrefixLoader, escape
 from jinja2.environment import Template
+from jinja2.exceptions import TemplateNotFound
 import uvicorn
 
 from .views.base import DatasetteError, ureg, AsgiRouter
@@ -745,7 +746,7 @@ class DatasetteRouter(AsgiRouter):
             path = "/" + path[len(base_url) :]
         return await super().route_path(scope, receive, send, path)
 
-    async def handle_404(self, scope, receive, send):
+    async def handle_404(self, scope, receive, send, exception=None):
         # If URL has a trailing slash, redirect to URL without it
         path = scope.get("raw_path", scope["path"].encode("utf8"))
         if path.endswith(b"/"):
@@ -754,7 +755,54 @@ class DatasetteRouter(AsgiRouter):
                 path += b"?" + scope["query_string"]
             await asgi_send_redirect(send, path.decode("latin1"))
         else:
-            await super().handle_404(scope, receive, send)
+            # Is there a pages/* template matching this path?
+            template_path = os.path.join("pages", *scope["path"].split("/")) + ".html"
+            try:
+                template = self.ds.jinja_env.select_template([template_path])
+            except TemplateNotFound:
+                template = None
+            if template:
+                headers = {}
+                status = [200]
+
+                def custom_header(name, value):
+                    headers[name] = value
+                    return ""
+
+                def custom_status(code):
+                    status[0] = code
+                    return ""
+
+                def custom_redirect(location, code=302):
+                    status[0] = code
+                    headers["Location"] = location
+                    return ""
+
+                body = await self.ds.render_template(
+                    template,
+                    {
+                        "custom_header": custom_header,
+                        "custom_status": custom_status,
+                        "custom_redirect": custom_redirect,
+                    },
+                    view_name="page",
+                )
+                # Pull content-type out into separate parameter
+                content_type = "text/html"
+                matches = [k for k in headers if k.lower() == "content-type"]
+                if matches:
+                    content_type = headers[matches[0]]
+                await asgi_send(
+                    send,
+                    body,
+                    status=status[0],
+                    headers=headers,
+                    content_type=content_type,
+                )
+            else:
+                await self.handle_500(
+                    scope, receive, send, exception or NotFound("404")
+                )
 
     async def handle_500(self, scope, receive, send, exception):
         title = None