Switched to using git URLs for submodules

closes #10265

- trying to balance ease of contributing with git urls not always working
- optimising for ease of contributing for now, but #10265 has other options just in case

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "content/themes/casper"]
 	path = content/themes/casper
-	url = https://github.com/TryGhost/Casper.git
+	url = git@github.com:TryGhost/Casper.git
 [submodule "core/client"]
 	path = core/client
-	url = https://github.com/TryGhost/Ghost-Admin.git
+	url = git@github.com:TryGhost/Ghost-Admin.git