Feature request: Better resource-handling (thumnails etc)

[nytamin]

Is your feature request related to a problem? Please describe.
Currently, the thumbnail-images are stored as base64-blobs along with the resource-JSON data, and is fetched synchronously along with the rest of the resource data.

Describe the solution you'd like
We should rework how we store and refresh resource data.

Note: Any code that handles "thumbnails" below should also be able to handle other things, like preview (webm) files, and others.

• Change how the thumbnails are stored on disk. Instead of base64-blobs we should store them as separate files.
• Change how the thumbnails are served to the client. Instead of going via IPC, they should be served from a http-endpoint.
• Change how the thumbnails are updated. A Resource-refresh should update only the resources themselves. The thumbnails should be updated asynchronously afterwards, and only when needed.

Additional context

This might potentially also fix the issue with SC crashing in certain cases (#133)