Bugfix: memoize slide sizes

We didn't store the fetched image size hash anywhere... Now we store it per instance and just fetch as needed on class level, as the size might change.
Add method for generating all slide images.

We will make use of this when the image sizes for the event change.
Clean up unused config related stuff
Store event config in db.

We store the per-event config in a serialized column in the db.
Comments
Allow setting the slide resolution.

We check if the resolution is supported and if so update the config hash.
Handle updated defaults on config load.

We merge DefaultConfig hash to the one read from the DB on read. This makes sure, that if we later add more keys to the DefaultConfig they will always be found from the hash returned by config method.
Allow setting the slide resolution per event.
Comments and whitespace
Add action to regenerate images

We now can request a background job to regenerate all images associated to a given event. This is useful after changing the slide image size or swapping the background.
Turn of synchronizing in test environment.

Previously running the test suite created huge amounts of websocket notifications to a local server due to the messages getting synced via redis. Now we turn off the inter-process sync in test environment.
Add the master svg file for error.png