Archive snapshots of websites locally (#672)

* Add basic HTML snapshots

* Implement asset list

* Add snapshot creation tests

* Add deletion tests

* Show file size

* Remove snapshots

* Create new snapshots

* Switch to single-file

* CSS tweak

* Remove auto refresh

* Show delete link when there is no file yet

* Add current date to display name

* Add flag for snapshot support

* Add option for disabling automatic snapshots

* Make snapshots sharable

* Document image variants

* Update README.md

* Add migrations

* Fix tests

bootstrap.sh

@@ -7,6 +7,8 @@ LD_SERVER_PORT="${LD_SERVER_PORT:-9090}"
 mkdir -p data
 # Create favicon folder if it does not exist
 mkdir -p data/favicons
+# Create assets folder if it does not exist
+mkdir -p data/assets
 
 # Generate secret key file if it does not exist
 python manage.py generate_secret_key