Email builder: actions out of the way, shortcuts that match the toolbar, and uploads you can file

A round of fixes to how the email builder feels while you edit.

• Block actions sit beside the email. Move, duplicate, save, who sees it and delete now appear in the margin next to a block instead of on top of its text.
• Text is edited on the canvas only. Headings no longer have a second text box in the side panel, and a columns block's panel is styling only: number of columns, gap, padding inside each column, vertical alignment and a background colour per column.
• The toolbar shows what's on. Click into bold, italic or underlined text, a list or a link and its button lights up. ⌘B / Ctrl+B, I, U and ⌘K / Ctrl+K do exactly what the buttons do.
• Open links in a new tab. The link dialog has an Open in a new tab option, on by default.
• Uploads say where they go. Uploading an image (in the email builder, the Content Library picker, forms and social posts) now asks which Content Library folder to save it in, with a progress bar and a confirmation.
• Scroll while dragging. Hold a block near the top or bottom of the canvas and it scrolls; the mouse wheel works too.
• Divider padding in columns. The side panel now shows a block's real left and right padding inside a column (0 by default), so changing it does what it says.