Based on https://github.com/ppd/syncthing-snap/
This supports uprading from old revisions, wich only support
syncthing running as a root daemon and also have non-standard
configuration locations.

Don't run root daemon by default:
Upstream recommends running syncthing as a normal user.
This snap supports both user-mode & root-mode.
For new installations, the daemon is disabled by default; old installations
will keep running without changes.

Expose syncthing command as app:
The syncthing command is now fully accessible from the CLI

Autostart for user-mode:
- Support interactive setup of autostart in the desktop session on first run (http-prompt)
- add commands for disabling & enabling autostart from the CLI

Accessing the GUI:
- Add Show Web UI desktop file
- Add show-ui command to display the web GUI
- Support accessing the UI for root-mode & user-mode
- expose root daemon GUI address in $SNAP_COMMON/gui-address

Other changes:
- upgrade to core20 base
- update version
- suppress upgrade notifications
- give syncthing process a lower priority (nice -n 9)
- add README.md