Add/loading state to unified nav

[getdave]

Changes proposed in this Pull Request

• Amend state shape to allow for tracking state of the API request.
• Add temporary loading UI <Spinner />.

Testing instructions

• Checkout this PR.
• Apply D49005-code
• Apply blog sticker
• Run calypso
• Visit http://calypso.localhost:3000/?flags=nav-unification.
• You should see a loading spinner appear in the sidebar whilst the Nav data is being loaded.
• The Nav should then appear once loading and loading state should disappear.

For debugging purposes you can force this UI state to appear by opening browser dev tools and typing state.adminMenu.requesting.isRequesting = true.

Fixes #45830

[matticbot]

Test live: https://calypso.live/?branch=add/loading-state-to-unified-nav

[matticbot]

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

Async-loaded Components (~150 bytes added 📈 [gzipped])

name                                          parsed_size           gzip_size
async-load-my-sites-sidebar-unified-switcher       +494 B  (+0.2%)     +150 B  (+0.3%)

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.

[getdave]

I'm going to amend this to follow a reducer/state pattern more closely aligned with client/state/plans/reducer.js.

[cpapazoglou]

Thanks Dave!
Tests pass and the UX is better than having no visual indicator 👍 !