added an about page with slow-web pitch

Shows instance stats, destination hash for subscribing, and explains
the slow web movement and how TinyWeb works. Destination hash is
stored in settings on startup so the about page can display it.

app.py

@@ -4,7 +4,7 @@ import threading
 import RNS
 from http.server import HTTPServer
 
-from db import init_db
+from db import init_db, set_setting
 from handlers import dispatch_request
 from gateway import GatewayState, GatewayHandler, GATEWAY_PORT
 
@@ -55,6 +55,7 @@ def main():
     )
 
     destination.announce()
+    set_setting("dest_hash", destination.hash.hex())
     start_gateway(reticulum)
 
     print(f"TinyWeb running!")