CPT: Enable type toggles for supported Jetpack versions

[aduth]

Related: #4502
Related: #6105
Blocked by: #4369

This pull request seeks to allow Jetpack sites running supported versions and having enabled the "Custom Content Types" feature to toggle the active custom post types for their site.

Testing instructions:

As noted above, this is blocked by #4369. It is not currently possible to change the active setting of a custom post type without these changes. Pending future release or with these changes applied manually to your Jetpack site (and meeting or changing the versionCompare version), verify that you can toggle custom post types on a site.

1. Navigate to the site writing settings
2. Select a site
3. Note that if your site is hosted on WordPress.com or has the Jetpack "Custom Content Types" module running on a supported Jetpack version, you are offered the option to toggle testimonials or portfolio projects
4. Note that the current toggle setting matches expectations
5. Toggle one or both of the post types and save your changes
6. Note that the save completes successfully and the sidebar navigation item is added or removed to reflect your change
7. Refresh the page to confirm that the setting change has persisted

Test live: https://calypso.live/?branch=update/cpt-jetpack-writing-settings

[timmyc]

shouldn't this be site.versionCompare( '4.2.0', '>=' ) && - since versionCompare returns a boolean?

[timmyc]

I tested this out against a site that is running 4.1.1 and I was still seeing the <CustomPostTypeFieldset />, I think it is related to my comment above. Using the toggles against my 4.1.1 site did not persist the changes, but toggling CPT for wpcom sites still works as expected.

[aduth]

@timmyc Yeah, that would make sense, since false >= 0. #6744 added a new isJetpackMinimumVersion selector, so I think I'll change to use that instead anyways.

[aduth]

Rebased and pushed an alternate solution using isJetpackMinimumVersion. @timmyc could you take this for another spin on your Jetpack site?

[timmyc]

Working as expected now. 🚢