Let users subscribe to new manuscripts for specific regions and timeframes

[nuest]

• regions are predefined: continents, oceans
• notification intervals predefined (weekly, monthly)
• combination of region and time period (using AND)
• emails contain two unsubscribe links (for the particular region/timeframe, from all subscriptions)
• lakes, rivers > https://github.com/kokoalberti/postgis-baselayers/tree/master/app/datasets/naturalearth/

See #14 for minimal feature.

• API sketch (discussed with @nuest ), see also Add spatial queries and filtering to the API #52
• UI mockup
• Tests
• Implementation