flatpak: Use intltool provided by shared-modules

Launchpad is currently dealing with DDoS issues. The shared-module
repository contains mirrors which can be useful for those who are
unable to build intltool.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/756>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "build-aux/shared-modules"]
+	path = build-aux/shared-modules
+	url = https://github.com/flathub/shared-modules

build-aux/org.gnome.Calendar.json

@@ -116,17 +116,7 @@
                 }
             ]
         },
-        {
-            "name": "intltool",
-            "cleanup": [ "*" ],
-            "sources": [
-                {
-                    "type": "archive",
-                    "url": "https://launchpad.net/intltool/trunk/0.51.0/+download/intltool-0.51.0.tar.gz",
-                    "sha256": "67c74d94196b153b774ab9f89b2fa6c6ba79352407037c8c14d5aeb334e959cd"
-                }
-            ]
-        },
+        "shared-modules/intltool/intltool-0.51.json",
         {
             "name" : "gcr",
             "buildsystem" : "meson",