Stories: moved __DEV__ flag to contact-info block availability declaration only

[mzorz]

This PR moved the __DEV__ flag up to a per-block granularity. Registering the jetpack editor code is no longer behind the dev flag, and now blocks that are in development can be added to the supportedJetpackBlocks dictionary with availability set to true|false or a flag (as is the case for the Contact Info block).

To test:

• Run with Update gutenberg-mobile with non-dev Story block WordPress-Android#13355 and verify opening a post containing a Story block renders as a Story block and not as an unsupported block.
• verify the Contact Info block only appears when wp.BUILD_GUTENBERG_FROM_SOURCE = true in WPAndroid's gradle.properties

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[aforcier]

Confirmed that with wordpress-mobile/WordPress-Android#13355 Story blocks are rendered in the editor and I can edit them.

Also confirmed that a contact info block only shows up as supported if wp.BUILD_GUTENBERG_FROM_SOURCE = true.

I'll let @jd-alexander or someone else also double check things but this looks good to me!

[etoledom]

Tested on iOS and the new block doesn't show as expected.
Code looks great and I trust @aforcier with Android side 🎉

[mzorz]

Thank you both @etoledom @aforcier ! 🙇