Nav Unification: Open wp-admin links on new tabs (Jetpack sites only)

[mmtr]

Changes proposed in this Pull Request

Ensures that all menu items linking to /wp-admin URLs are marked as external and opened in new tabs for Jetpack sites.

This improves the nav unification experience for Jetpack sites, since WP Admin is considered a separate area on those sites.

Testing instructions

• Spin up a Jetpack site running the update/admin-menu-jetpack-sites branch (Admin Menu: Mimic existing Calypso menu on Jetpack sites jetpack#18921).
• Use the Calypso live link below, and switch to that Jetpack site.
• Make sure all menus linking to /wp-admin URLs (Feedback, Customize, Import, Export, WP Admin) are marked as external and they open pages in a new tab.
• Make sure there are no regression on Simple/Atomic sites (all menus should open in the same tab except when Happychat is active).

Fixes (partially) #50438

[matticbot]

Test live: https://calypso.live/?branch=update/sidebar-unified-jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Async-loaded Components (~87 bytes added 📈 [gzipped])

name                                         parsed_size           gzip_size
async-load-calypso-my-sites-sidebar-unified       +364 B  (+0.2%)      +87 B  (+0.2%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[cpapazoglou]

It works well from user perspective. I left some comments, let me know your thoughts!

[cpapazoglou]

Thanks for refactoring this! It looks and works great now and code is simplified! LGTM, 🚢 !