WP Stories: Only show Story block in GB picker if supported by the site

[aforcier]

I noticed that the Story block shows up in the Gutenberg picker for all sites - including non-Jetpack self-hosted sites and sites using older Jetpack versions.

To fix it I added the same check we use in the bottom sheets to determine whether the story feature is available.

Note: This PR should be included in the 16.3 release, so will require a new beta. I'm targeting fix/stories-for-16.3 for now though because there are one or two other issues that need to be resolved that may as well all be bundled into one updated beta cut.

To test:

• For a WordPress.com or Jetpack 9.1+ site, check that the Story block shows up in the block picker
• For a self-hosted non-Jetpack site, or a site running an older version of Jetpack, check that the Story block does not show up in the block picker

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[mzorz]

Good call, thanks for this change @aforcier !
LGTM