Add additional migration notice for GB 1.0 users

[nickcernis]

To encourage Genesis Blocks 1.0 users to update to Genesis Blocks 1.1.

@susannelson discovered that Genesis Blocks 1.0 users who click the “Install and Activate Genesis Blocks” button at Atomic Blocks → Migrate see a “you do not have permission to access this page” message when they are redirected to the Genesis Blocks migrate page. That page only exists if they are running Genesis Blocks 1.1, and the current migration process only activates an existing copy of Genesis Blocks — it does not attempt to update it to the latest version from WP.org.

This PR adds some additional text to prompt such users to update.

How to test

Review text at Atomic Blocks → Migrate.

Documentation

@susannelson Does the new text help to address this issue, or do you recommend further changes?

Notes

An alternative to displaying text is to attempt to detect the presence of Genesis Blocks, store its version, and then change the button action:

• If GB version is <1.1: update the plugin to GB 1.1+ (if safe/possible?), activate GB, then redirect to the migration page. OR: show a prompt instead of a button, and link to the Plugin Update page.
• If GB version is >=1.1: activate GB and redirect to the migration page [current behaviour].

We chose to add some text because it's an edge case (there are more people using AB alone than AB plus GB), but happy to adjust this or pick another approach if needed.

[dreamwhisper]

I ran into this earlier and was just coming to report! Thanks for the update!

[johnstonphilip]

@nickcernis This looks good. But I am seeing a typo in the screenshot. It looks like I (my fault) lost the word "has" in "Atomic Blocks has been renamed to Genesis Blocks" here during merge conflict fixes:

https://github.com/studiopress/atomic-blocks/commit/c8f1d2bb73d64b5fb6a9e5395563f9d806352df0

Not sure if this is the place to fix that or not, but we should add that word "has" back in again.

[nickcernis]

Thanks for the review and good catch, @johnstonphilip — fixed in https://github.com/studiopress/atomic-blocks/pull/410/commits/0aecd8b69df4081b8a99b56eb31a03708badd468.