Changing attachment download behaviour to save disk space

One issue that came up frequently in the forum is that Joplin's data can be very large, especially when the note collection includes many attachments (images, PDFs, etc.). This happens in particular when using the web clipper a lot, as each web page usually has many images included.

The recent versions of Joplin (Desktop, mobile and CLI) attempt to mitigate this issue by providing an option to change how attachments are downloaded during synchronisation.

The default option is to download all the attachments, all the time, so that the data is available even when the device is offline. However, more importantly, there's now the option to download the attachments manually, by clicking on it, or automatically, in which case the attachments are downloaded only when a note is opened.

These changes should help saving disk space and network bandwidth, especially on mobile.