Rename snapshot identifier in json output from "number" to something more fitting

AFAIK Commit 89952d49 is intended to fix #47 (closed).

However in turn it broke reading list of snapshots in foris-controller, thus breaking reForis snapshots page too.

In listing snapshots, the snapshot number does not necessary need to be a number anymore, especially with remote snapshots. So let's put ti in quotes so it is always a string.

I see the reasoning behind the fix, but I wouldn't call it "number" anymore.

Based on its name, anyone could assume numeric value and try to validate it like number (actually as integer in foris-controller) or process it like number, which would fail on arbitrary string.

So I would suggest renaming it to "snapshot_id" or something more fitting.