swarm-grafana: put the queue panels that pair next to each other

The pending-bytes panel described itself as the leading indicator for
the slow-consumer stat 'beside it', and then sat alone in its own row
with eighteen grid units of empty space. A description that tells the
reader to look beside it, where nothing is, is worse than no
description.

Slow consumers and pending bytes are now adjacent in the top row, and
the two JetStream panels share the second. No gaps.

nix/host-modules/swarm-grafana/dashboards/queue.json

@@ -222,8 +222,8 @@
       "gridPos": {
         "h": 4,
         "w": 6,
-        "x": 18,
-        "y": 0
+        "x": 0,
+        "y": 4
       },
       "targets": [
         {
@@ -289,7 +289,7 @@
         "h": 8,
         "w": 24,
         "x": 0,
-        "y": 4
+        "y": 8
       },
       "targets": [
         {
@@ -354,8 +354,8 @@
       "gridPos": {
         "h": 4,
         "w": 6,
-        "x": 0,
-        "y": 12
+        "x": 18,
+        "y": 0
       },
       "targets": [
         {
@@ -412,10 +412,10 @@
         "uid": "@datasourceUid@"
       },
       "gridPos": {
-        "h": 8,
-        "w": 24,
-        "x": 0,
-        "y": 16
+        "h": 4,
+        "w": 18,
+        "x": 6,
+        "y": 4
       },
       "targets": [
         {