add support for direct onion downloads

[n8fr8]

• for sharing, add manifest at well-known endpoint
• if present use that to download files
• save to "Files"
• add in app download monitor / tab

[n8fr8]

support for customization of shared onion links to not have ".onion" in them, and be specific to the app scheme/handler

[tladesignz]

• for sharing, add manifest at well-known endpoint

https://github.com/onionshare/onionshare-shared/issues/4