feat: webdav integration

This patch provides an optional way to integrate an external WebDAV server so that the super-productivity container can serve as a WebDAV server with base url http://localhost/webdav/ . It includes the following changes:

**Replace the default nginx config file**
Besides serving the web app, the new nginx config file also forwards all the requests with paths starting with "/webdav/" to a backend WebDAV server specified by the environment variable WEBDAV_BACKEND. Note that during forwarding, the path prefix "/webdav" will be removed.

**Use hacdias/webdav as default WebDAV backend server**
The docker-compose.yaml provides an example setup to use the docker image [hacdias/webdav](https://github.com/hacdias/webdav) as the WebDAV backend server. An example for the configuration of the WebDAV server is also provided in webdav.yaml.

docker-compose.yaml

@@ -0,0 +1,17 @@
+services:
+  sp:
+    image: johannesjo/super-productivity:latest
+    ports:
+      - 80:80
+    environment:
+      - WEBDAV_BACKEND=http://webdav # Optional
+
+  # Optional: WebDAV backend server
+  # (used with the WEBDAV_BACKEND environment variable)
+  webdav:
+    image: hacdias/webdav:latest
+    ports:
+      - 81:80
+    volumes:
+      - ./webdav.yaml:/config.yml:ro
+      - ./data:/data