Modernise discourse-kanban-theme implementation (#35)

Highlights include:
- Glimmer components / @Tracked
- Use new renderInOutlet API
- New modal API
- Avoid modifyClass on discovery controllers
- Add support for tag and tag/category routes
- Add 'copy link' button to boards
- Add 'empty state' UX to prompt for configuration
- Add system specs

.discourse-compatibility

@@ -1,3 +1,4 @@
+< 3.1.0.beta2-dev: c78a3998cdafabde7b9fa2e99ddab8ad2449c994
 3.1.999: 62b3115262fe50b523c0b1b4026be75e2b25f64a
 3.1.0.beta1: 244c2c47e0131bbb9f5be26990010e8ce37471f3
 2.7.0.beta3: 16918e67b2b16cb91a2dee1caea1bc96c317a5d0

.prettierrc

@@ -1 +1,17 @@
-{}
+{
+  "plugins": ["prettier-plugin-ember-template-tag"],
+  "overrides": [
+    {
+      "files": "*.gjs",
+      "options": {
+        "parser": "ember-template-tag"
+      }
+    },
+    {
+      "files": "*.gts",
+      "options": {
+        "parser": "ember-template-tag"
+      }
+    }
+  ]
+}

about.json

@@ -9,5 +9,7 @@
   "maximum_discourse_version": null,
   "assets": {},
   "color_schemes": {},
-  "learn_more": "https://meta.discourse.org/t/beginners-guide-to-using-discourse-themes/91966"
+  "modifiers": {
+    "svg_icons": ["user-minus"]
+  }
 }

common/common.scss

@@ -62,6 +62,13 @@
     overflow: hidden;
     background: var(--primary-low, $primary-low);
 
+    &.kanban-empty-state {
+      padding: 10px;
+      text-align: center;
+      justify-content: center;
+      gap: 20px;
+    }
+
     &.accept-drag {
       background: var(--primary-medium, $primary-medium);
     }
@@ -111,7 +118,6 @@
     .topic-card {
       display: block;
       color: var(--primary-medium, $primary-medium);
-      cursor: pointer;
 
       &.topic-unseen {
         color: var(--primary-medium, $primary-medium);