[form-builder] Normalize blocks received from custom paste handler

[skogsmaskin]

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe:

Does this change require a documentation update? (Check one)

• Yes
• No

Current behavior
Blocks returned from a custom paste handler is not normalized. This can lead to errors (missing keys etc). We should totally normalize them, before inserting them into the block array.

Description
Blocks are now normalized so that they are always "right", and the end user doesn't have to worry about it.

Note for release
Fix a bug where blocks returned from a custom paste handler in the portable text editor was not normalized.

Checklist

• I have read the Contributing Guidelines
• The PR title includes a link to the relevant issue
• The PR title is appropriately formatted: [some-package] PR title (#123)
• The code is linted
• The test suite is passing
• Corresponding changes to the documentation have been made