Multiple pages per board

[x] Each board should have multiple pages, with next/previous page support
[x] Should all pages be loaded (just not displayed)?
- "Yes, load everything" for now, for fast page flipping
- "No, just load single page" for history view, as this tends to be bigger and slower
- [ ] Partial load if database gets too big
[x] Manual entry of page number
[x] Add page at the end, and ideally in the middle of sequence
[x] Duplicate page
[ ] Delete/archive page
- [ ] If page has no history, or no history since duplication, then delete; otherwise archive
[ ] Show deleted/archived pages toggle.
- Issue: don't want to subscribe to objects in deleted pages unless toggle is on. Maybe restrict to objects in undeleted pages by default in subscription?
[ ] Undeleting page: delete button can be a toggle when in "show deleted pages" view
[ ] Re-order pages
[ ] Should we have a thumbnail view of pages? Who is responsible for updating this?
[ ] Hierarchy of pages (replacement for #20?)
[ ] Page titles (after #6)
[x] Move grid setting to page instead of board
[x] Help and shortcuts for page tools (need layout for .top.horizontal)

edemaine / cocreate