Convert logo and signature to actual file upload fileds #193

Is your feature request related to a problem? Please describe.
The settings for logo and signature currently require a base64 url encoded string (since they are used for PDF creation).

Describe the solution you'd like
Both fields should be converted to Filaments file upload input field. Uploaded images can be replaced or deleted. Bonus: A little preview image showing the current upload.

Additional context
File uploads were first introduced here #192.