Add gallery generator

[dannystaple]

So take a buncha pictures in a folder, generate a placeholder post to show their thumbs (one style), or an article with responsive full size imageS (other).
I can hit that button, then fill details in a form, and go. Can I make it folder selection?

[dannystaple]

Thoughts:

• The post can be a normal blog post - doesn't need to be some specific separate type.
• Flag for thumbs or responsive with click (optional).
• Space for alt tags, and format for captions on the images.
• Not interested in scrolly coverview type things.
• Opinionated - goes in the galleries folder, not assets (save that confusion - assets should be for the UI/style, not content images).
• No fancy yaml bits for descriptions - these are just in the template between the images in markdown.