Add server banner to web app, add GET /api/v2/instance to REST API (#19294)

Cherry-picked d2528b26b6

Conflicts:
- `app/serializers/initial_state_serializer.rb`:
  Upstream changed stuff, we had extra attributes.
  Applied upstream changes while keeping our extra attributes.
- `app/serializers/rest/instance_serializer.rb`:
  Upstream actually moved that to `app/serializers/rest/v1/instance_serializer.rb`,
  so updated that file by keeping our extra attributes, and took upstream's
  version of `app/serializers/rest/instance_serializer.rb`.
- `spec/views/about/show.html.haml_spec.rb`:
  Took upstream's version.

app/javascript/styles/mastodon/components.scss

@@ -7949,3 +7949,85 @@ noscript {
     }
   }
 }
+
+.server-banner {
+  padding: 20px 0;
+
+  &__introduction {
+    color: $darker-text-color;
+    margin-bottom: 20px;
+
+    strong {
+      font-weight: 600;
+    }
+
+    a {
+      color: inherit;
+      text-decoration: underline;
+
+      &:hover,
+      &:active,
+      &:focus {
+        text-decoration: none;
+      }
+    }
+  }
+
+  &__hero {
+    display: block;
+    border-radius: 4px;
+    width: 100%;
+    height: auto;
+    margin-bottom: 20px;
+    aspect-ratio: 1.9;
+    border: 0;
+    background: $ui-base-color;
+    object-fit: cover;
+  }
+
+  &__description {
+    margin-bottom: 20px;
+  }
+
+  &__meta {
+    display: flex;
+    gap: 10px;
+    max-width: 100%;
+
+    &__column {
+      flex: 0 0 auto;
+      width: calc(50% - 5px);
+      overflow: hidden;
+    }
+  }
+
+  &__number {
+    font-weight: 600;
+    color: $primary-text-color;
+  }
+
+  &__number-label {
+    color: $darker-text-color;
+    font-weight: 500;
+  }
+
+  h4 {
+    text-transform: uppercase;
+    color: $darker-text-color;
+    margin-bottom: 10px;
+    font-weight: 600;
+  }
+
+  .account {
+    padding: 0;
+    border: 0;
+  }
+
+  .account__avatar-wrapper {
+    margin-left: 0;
+  }
+
+  .spacer {
+    margin: 10px 0;
+  }
+}