Fix Permalink editor and sibling block inserter

This bugfix release fixes two issues in 3.1.0:

• Fix permalink editor not appearing. (#7467)
• Fix sibling block inserter not working in Firefox and Safari. (#7508)

v1.0.0: Merge pull request #2589 from WordPress/update/1.0.0

If you're trying to run this, it requires WP 4.8 or WP 4.9