AdminUX Overhaul (#2409)

- Extensions now have their own pages
- The API for extensions to register permissions and settings has been overhauled via the `flarum/admin/utils/ExtensionData` util
- An extension grid has been added as a widget to the Dashboard page

less/admin/ExtensionWidget.less

@@ -0,0 +1,93 @@
+.ExtensionsWidget {
+  background-color: @body-bg;
+  padding: 0;
+}
+
+.ExtensionsWidget-list {
+  > .container {
+    padding: 0;
+    background-color: @body-bg;
+
+    .ExtensionList-Category {
+      background: @control-bg;
+      padding: 20px 0 20px 20px;
+      margin-bottom: 20px;
+      border-radius: @border-radius;
+
+      .ExtensionList-Label {
+        margin-top: 0;
+        color: @muted-color;
+      }
+    }
+
+    .ExtensionGroup {
+      margin-bottom: 20px;
+
+      h3 {
+        color: @muted-color;
+        text-transform: uppercase;
+        font-size: 12px;
+        margin: 0 0 10px;
+      }
+    }
+
+    .ExtensionList {
+      padding: 0;
+      list-style: none;
+      display: grid;
+      grid-gap: 10px;
+      grid-template-columns: repeat(auto-fit, 90px);
+      margin-bottom: 0;
+
+      > li {
+        text-align: left;
+        position: relative;
+        display: block;
+      }
+    }
+  }
+
+  .ExtensionListItem.disabled {
+    .ExtensionListItem-title {
+      opacity: 0.5;
+      color: @muted-color;
+    }
+
+    .ExtensionListItem-icon {
+      opacity: 0.5;
+    }
+  }
+
+  .ExtensionListItem {
+    transition: .15s ease-in-out;
+
+    &:hover {
+      transform: scale(1.05);
+    }
+
+    .ExtensionListItem-title {
+      display: block;
+      text-align: center;
+      margin-top: 5px;
+      color: @text-color;
+    }
+
+    a:hover {
+      text-decoration: none;
+    }
+  }
+}
+
+.ExtensionIcon {
+  width: 90px;
+  height: 90px;
+  background: @control-bg;
+  color: @control-color;
+  border-radius: 6px;
+  display: inline-flex;
+  font-size: 45px;
+  text-align: center;
+  align-items: center;
+  justify-content: center;
+  vertical-align: middle;
+}