should have new content notify spot on subscribed communities

[mydearxym]

see https://github.com/coderplanets/coderplanets_web/issues/354

details:

1. add field to every community table which tracks last content published timestamp
2. add json-field to login user's C11N subscribed communities which tracks every community's last read timestamp
3. the whole logic to frontend should abstract to a 'has_new_content' field on each subscribed-community
4. the whole logic should done by MIDDLEWARE