Performance/Webpack: Introduces more aggressive code-splitting and other perf improvements (#18544)

* Performance/Webpack: Introduces more aggressive code-splitting and other perf improvements
- Introduces dynamic imports for built-in plugins
- Uses dynamic imports for various packages (rst2html, brace)
- Introduces route-based dynamic imports
- Splits angular and moment into separate bundles

public/app/features/users/UsersActionBar.tsx

@@ -68,7 +68,7 @@ export class UsersActionBar extends PureComponent<Props> {
             </a>
           )}
           {externalUserMngLinkUrl && (
-            <a className="btn btn-primary" href={externalUserMngLinkUrl} target="_blank">
+            <a className="btn btn-primary" href={externalUserMngLinkUrl} target="_blank" rel="noopener">
               <i className="fa fa-external-link-square" /> {externalUserMngLinkName}
             </a>
           )}

public/app/features/users/__snapshots__/UsersActionBar.test.tsx.snap

@@ -86,6 +86,7 @@ exports[`Render should show external user management button 1`] = `
     <a
       className="btn btn-primary"
       href="some/url"
+      rel="noopener"
       target="_blank"
     >
       <i