Add option to ignore subfolders for auto-upload

### Is your feature request related to a problem? Please describe.
I'm currently struggling to set up automatic upload for images that I saved to the internal storage from within signal. Those images get saved in \pictures , but not in a subfolder like eg. telegram (\pictures\telegram). This arises a problem, as the only option present to me is to sync the whole \pictures folder. This will end up syncing all images, but will create subfolders for telegram etc. in nextcloud - something I try to avoid. I would like to have all my images in one nextcloud folder.

### Describe the solution you'd like
I think the simplest and best solution to this problem would be to add an option to ignore subfolders when setting up automatic upload for a specific folder.
This way it would be possible to enable automatic upload for each (sub-)folder separately and as a result to have all images in one folder on ones nextcloud.

### Describe alternatives you've considered
I've thought about an option to upload all images of subfolders into the directory specified in the auto upload settings of the main folder, but I don't think that's a good solution as it may lead to a lot of unnecessary file conflicts and would also reduce the control one has over choosing/excluding specific subfolders in comparison to my first solution.

### Additional context
I recognize that my specific case with saving images from signal is something that can also be worked on from their side.
Nonetheless I think this option has more use cases and would greatly add to the ability to customize automatic uploading to Nextcloud from Android.