Update to new design for avatars: individual/group icons/colors

And two initials.

ts/styleguide/LeftPaneContext.tsx

@@ -0,0 +1,25 @@
+import React from 'react';
+import classNames from 'classnames';
+
+interface Props {
+  /**
+   * Corresponds to the theme setting in the app, and the class added to the root element.
+   */
+  theme: 'light-theme' | 'dark-theme';
+}
+
+/**
+ * Provides the parent elements necessary to allow the main Signal Desktop stylesheet to
+ * apply (with no changes) to messages in the Style Guide.
+ */
+export class LeftPaneContext extends React.Component<Props> {
+  public render() {
+    const { theme } = this.props;
+
+    return (
+      <div className={classNames(theme || 'light-theme')}>
+        <div className="gutter">{this.props.children}</div>
+      </div>
+    );
+  }
+}