Cleanup the Gallery block issues

The gallery block in Gutenberg 3.0.x seems to be forgotten a bit, there are multiple issues already created for this but I believe it's best to reference all of them in one place and we can tackled them one by one for reference. This has been rather a blocker for an upgrade for us for a while.

Related issues in my opinion's priority:

• Images are not linked with core/gallery block 🐛 Images are not linked with core/gallery block Active
• Gallery only vertical presentation 💬 Gallery only vertical presentation Active
• How to add a alternative text on images in gallery ? 💬 How to add a alternative text on images in gallery ? Active
• Derivatives images not generated 🐛 Derivatives images not generated Active
• Gutenberg wrap text and images 💬 Gutenberg wrap text and images Active

There are more issues but I assume these are the blockers/higher priorties