[Feature Request] Settings local backup #1414

The app has a good amount of customization and configuration going on at the user's end, so I figured it would be good to have a feature to backup settings—as a local file one can download, or to the cloud.

This way, it's easier to get our configuration setup when we download the app on a new device, or when we have to delete app data (I had to delete all feeds because I had a fresh opml file to import, and deleting app data was the only way I could delete them quickly because deleting the categories was taking so long, so I think that needs some optimization as well—speed of deleting feeds—either individually, or in bulk).