Reader Editor: Use the SitesDropdown component

[DustyReagan]

Related to https://github.com/Automattic/loop/issues/455

Proposed Changes

• Use the SitesDropdown component for Quick Posts.

Before	After

Why are these changes being made?

• The SitesDropdown component has many helpful features built-in to help users find the site they want to post to.
• We use SitesDropdown in other places, such as /me/account, so reusing it provides a consistent experience.

Testing Instructions

• Use Calypso Live and go to /reader?flags=reader/quick-post
• The site selection drop-down should work as it did before, and you should be able to post to your sites using the editor and the "Post" button.
• Double-check that nothing will leak outside of the feature flag. (It shouldn't)

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-134542

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-134542&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-134542&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/quick-post-site-selector on your sandbox.

[matticbot]

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

Sections (~425 bytes added 📈 [gzipped])

name    parsed_size           gzip_size
reader      +1993 B  (+0.1%)     +425 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

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.