List mirrors of updates sites right after the original site.

The order that update sites matter when multiple update sites provide
the same files.  This is how the Java-8 update site overwrites files
provided by the ImageJ and Fiji update sites.

At the moment, the ImageJ, Fiji, and Java-8 mirror site are listed
after the original.  If someone selects the ImageJ and Fiji mirrors
but not the Java-8 mirror, then the Java-8 overwrite is replaced by
the mirrors.  To avoid this, this commit moves the ImageJ and Fiji
update sites under the original update site.  See
https://forum.image.sc/t/errors-with-european-mirrors-for-fiji-and-imagej/103355

sites.yml

@@ -8,6 +8,15 @@ sites:
     maintainers:
       - "[ImageJ2 developers](https://imagej.net/people#imagej2)"
 
+  - name: "ImageJ (Europe mirror)"
+    id: "ImageJ-micron.ox.ac.uk"
+    url: "https://downloads.micron.ox.ac.uk/fiji_update/mirrors/imagej/"
+    description: >-
+      European mirror for the ImageJ update site.  Replace the ImageJ
+      update site with this one for faster updates in Europe.
+    maintainers:
+      - "[ImageJ2 developers](https://imagej.net/people#imagej2)"
+
   - name: "Fiji"
     id: "Fiji"
     url: "https://update.fiji.sc/"
@@ -18,6 +27,15 @@ sites:
     maintainers:
       - "[Fiji developers](https://imagej.net/people#fiji)"
 
+  - name: "Fiji (Europe mirror)"
+    id: "Fiji-micron.ox.ac.uk"
+    url: "https://downloads.micron.ox.ac.uk/fiji_update/mirrors/fiji/"
+    description: >-
+      European mirror for the Fiji update site.  Replace the Fiji
+      update site with this one for faster updates in Europe.
+    maintainers:
+      - "[Fiji developers](https://imagej.net/people#fiji)"
+
   - name: "Fiji-Legacy"
     id: "Fiji-Legacy"
     url: "https://sites.imagej.net/Fiji-Legacy/"
@@ -38,24 +56,6 @@ sites:
     maintainers:
       - "[ImageJ2 and Fiji developers](https://imagej.net/people)"
 
-  - name: "ImageJ (Europe mirror)"
-    id: "ImageJ-micron.ox.ac.uk"
-    url: "https://downloads.micron.ox.ac.uk/fiji_update/mirrors/imagej/"
-    description: >-
-      European mirror for the ImageJ update site.  Replace the ImageJ
-      update site with this one for faster updates in Europe.
-    maintainers:
-      - "[ImageJ2 developers](https://imagej.net/people#imagej2)"
-
-  - name: "Fiji (Europe mirror)"
-    id: "Fiji-micron.ox.ac.uk"
-    url: "https://downloads.micron.ox.ac.uk/fiji_update/mirrors/fiji/"
-    description: >-
-      European mirror for the Fiji update site.  Replace the Fiji
-      update site with this one for faster updates in Europe.
-    maintainers:
-      - "[Fiji developers](https://imagej.net/people#fiji)"
-
   - name: "Java-8 (Europe mirror)"
     id: "Java-8-micron.ox.ac.uk"
     url: "https://downloads.micron.ox.ac.uk/fiji_update/mirrors/java-8/"