Introduce upload limit .env option

[ssddanbrown]

Describe the feature you'd like

Introduce a new .env option to define a limit for file upload size.

This is already commonly enforced at a webserver and often PHP level but we should allow this to be configured at an app level.

This should replace and automatically define the existing JavaScript upload limit option:
https://www.bookstackapp.com/docs/admin/upload-config/#file-upload-limit

Will need to communicate this change as it would likely affect existing instances.

Need to think of a sensible default to enforce.