feat(linkding): Add linkding stack

Bookmarking software similar to shaarli but a bit more featureful. And
not written in php, thankfully.

roles/linkding/templates/upstream.json.j2

@@ -0,0 +1,38 @@
+{
+    "@id": "{{ stack_name }}_upstream",
+{% if server_domain is not undefined and not none %}   
+    "match": [
+        {
+            "host": [
+{% if subdomain_alias is not undefined and not none %}
+                "{{ subdomain_alias }}.{{ server_domain }}"
+{% else %}
+                "{{ stack_name }}.{{ server_domain }}"
+{% endif %}
+            ]
+        }
+    ],
+{% else %}
+    "match": [
+        {
+            "path": [
+{% if subdomain_alias is not undefined and not none %}
+                "/{{ subdomain_alias }}*"
+{% else %}
+                "/{{ stack_name }}*"
+{% endif %}
+            ]
+        }
+    ],
+{% endif %}
+    "handle": [
+        {
+            "handler": "reverse_proxy",
+            "upstreams": [
+                {
+                    "dial": "{{ stack_name }}_app:80"
+                }
+            ]
+        }
+    ]
+}