#7462764 Add promote posts button to stats entries, with feature flag

Proposed Changes

Hi Team, this is my first calypso PR and part of a series to add entrypoints in calypso for a new post promotion widget. Which will allow wordpress.com users to pay for advertising and serve the posts as ads on various A8C products.

Info specifically on these tasks here (pbpazC-3cN-p2) and the wider project thread here (pbpazC-2Pd-p2)

The majority of this feature is done outside of WP.com as a standalone widget, here we only need to add buttons to Calypso to show the widget and to pass it the site & post id’s.

Note: We will be iterating on a bunch of PRs to complete this feature, but to keep these smaller we will be merging one-by-one under a feature flag.

Here are our provided designs:

And here is the screenshot in my development environment:

Testing Instructions

• Enable the feature flag promote-posts either on startup or via url using the ?flags=promote-post query string on the following urls
• Visit the stats page for your favourite blog for example: http://calypso.localhost:3000/stats/day/something.wordpress.com
• In the posts and pages section, hover over a post and you should see a “speaker” icon like in the designs abover
  
• clicking the button should open the widget. However, this is not in production yet so without setting up your environment you can check the network tab and notice that it’s trying to hit the script widget.js – this will repeatedly fail. In a working dev environment – it only ever loads the script once.
• Test the above Steps without the feature flag enabled, ensure widget.js is not loaded in the f12 -> network tab and that the speaker button does not appear on hover.

Without local instance of the widget running

With local instance of the widget running

Pre-merge Checklist

• 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 you sent any new strings for translation (PCYsg-1vr-p2) ASAP?

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