feat(sterni/external): add flipdot-gschichtler

We'll need this in depot in order to import my infra, as I run
flipdot-gschichtler for the OpenLab at the moment. Importing the
repository into the tree is not really an option, as it should stay in
the GitHub organization. Additionally, it doesn't currently really have
a license, but it is very possible it'll end up being AGPL.

The whole thing is quite tame and has (anecdotically) never broken on a
nixpkgs channel update. A new niv sources area is created to avoid
cluttering the global one and having these sources in a gc root
permanently.

Change-Id: If49c6c0bf59bda9a90ca5cc405423affe52d0665
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7288
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

users/sterni/external/sources.json

@@ -0,0 +1,14 @@
+{
+    "flipdot-gschichtler": {
+        "branch": "master",
+        "description": "send text to the flipdots, orderly queued",
+        "homepage": "https://flipdot.openlab-augsburg.de",
+        "owner": "openlab-aux",
+        "repo": "flipdot-gschichtler",
+        "rev": "93683a7fff04e167963b70a8906f982567646501",
+        "sha256": "134kgmlv63vzdvc3lr0rys55klmzip7qpfnyzssahihp4mjyyq16",
+        "type": "tarball",
+        "url": "https://github.com/openlab-aux/flipdot-gschichtler/archive/93683a7fff04e167963b70a8906f982567646501.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}