Change how hostname/devices work

A few things we should consider changing:

- [x] Every device/installation should have a unique ID (a UUID) that does not change if the hostname is changed.
- Completed in https://github.com/ActivityWatch/aw-server/pull/61 and https://github.com/ActivityWatch/aw-server-rust/pull/69
- [ ] If a bucket is created with a special hostname/UUID "local" or similar then it should default to the hostname/UUID of the running instance.
- Or it should be the responsibility of the watcher to retrieve the UUID from `/api/0/info` and use it during bucket creation. Although defaulting to the local value seems like better API ergonomics (and would maybe make migration easier).
- As an example, it is required for the web watcher to work properly with sync (see https://github.com/ActivityWatch/activitywatch/issues/307)
- [ ] Additionally, we might want to move the hostname (later UUID) bucket attribute into the `data` field of buckets. This would impose specific semantics on the data field of buckets (which we currently lack) so might need further discussion (i.e. which data attributes are reserved for internal use in aw-server?)

Originally discussed here: https://github.com/ActivityWatch/aw-server-rust/issues/61#issuecomment-531687577