Add flood datasets

[annehaley]

This PR adds 9 datasets to datasets.json. Running python manage.py populate in the Django container will make these datasets available on the map.

• 9 inch sea level rise
• 9 inch 10 year flood
• 9 inch 100 year flood
• 21 inch sea level rise
• 21 inch 10 year flood
• 21 inch 100 year flood
• 36 inch sea level rise
• 36 inch 10 year flood
• 36 inch 100 year flood

The below screenshot shows the 9 inch 10 year Flood Dataset:

[annehaley]

@johnkit would you mind reviewing again? I have rebased this on master to include #15 and merged #18 into this.

With the changes from #18, the flood datasets won't get blurry at high zoom levels. You would need to re-run the populate script to change the database and see the difference.

[johnkit]

Retested the rebased version and all good (no blur when zoomed in).