Email builder: one toolbar at a time, Library-only images, and HTML emails imported with their images

More polish for the email builder, and a proper way in for HTML emails made elsewhere.

• Only the block you point at shows its toolbar. A block inside a column shows its own actions, not the column block's too; point at the column block's edge or gap to get its actions. The toolbar stays up while you move towards it, even across another block, and you never see more than two: the selected block's and the one you are pointing at.
• Every image goes through the Content Library. Add image and Replace both open the Library, where you can upload too, so every image in an email is one you can find again.
• Tidier side panel. Mobile and visibility settings are at the bottom, Delete lives only on the block's own toolbar, and the text block's panel is gone: alignment, line spacing, colour and clean-up are on the toolbar above the canvas.
• HTML emails are sent as written. Style blocks, media queries, Outlook conditional comments and the doctype are kept; scripts, forms, frames and event handlers are still removed.
• Import an HTML email with its images. Drop a .zip, a folder, or the .html with its image files. The images are saved to a new Content Library folder, the email is pointed at those copies, and you see what was found, what is missing and what was removed before anything uploads.