ChecklistShow: Decouple tasks prop

[ockham]

Move the responsibility to provide a tasks array prop to the Checklist prop from the ChecklistShow wrapper to its consumer. The rationale is that for https://github.com/Automattic/wp-calypso/projects/70, the logic providing that prop will diverge for JP sites from how we're doing it for WP.com ones, since in the Jetpack case, we want to display 'pending' state for some steps (spinners), which we'll infer from selectors (essentially plugin installation state, see #25935). To that end, we want to be able to transparently extend the tasks array in the connect() that is relevant for the JP case.

Note that this PR changes behavior: the checklist for JP sites is now only available at /plans/my-plan/:site, whereas /checklist/:site will no longer show a checklist for JP sites (only for WP.com).

Testing Instructions

Verify that the checklist still works for WP.com sites

• Navigate to http://calypso.localhost:3000/checklist/<wpcomSite>
• Verify that the loading state (pulsating placeholders) works fine: no errors, and it's only transient 🙂
• Try completing a couple of steps. Verify that they are correctly marked as complete afterwards, and that their completions status persists across a reload.
• Verify that you can also tick off tasks even without actually completing them by clicking the circle on the left hand side (which will turn into a green checkmark). Verify that this also persists across a reload.
• Try switching to a JP site -- you'll see a message "Checklist not available for this site" (We will add a redirect to /plans/my-plan/<jpSite> going forward.)

Verify that the checklist still works for Jetpack sites

• Navigate to http://calypso.localhost:3000/checklist/<jpSite>
• Verify that the loading state (pulsating placeholders) works fine: no errors, and it's only transient 🙂
• Try completing a couple of steps. Verify that they are correctly marked as complete afterwards, and that their completions status persists across a reload.
• Verify that propType errors in the console are pre-existing on master.
• Try switching to a WP.com site, and verify that no checklist is being displayed.

[matticbot]

Test live: https://calypso.live/?branch=update/checklist-show-decouple-tasks

[sirreal]

Rebased (new node version)

[ockham]

Rebased

[sirreal]

I'm not sure this aligns with the direction outlined in #26015

Before reviewing this fully, I'm going to do some exploration to see what implementing those changes might look like.

[sirreal]

I'd like to go with #26216 over this, it's about ready 😁