Create from…: entry points on lists and list rows #13

Part of #10 — "Create from…" (materialize) — turn messages, lists, rows and documents into new lists/documents/posts

Web behaviour: Create-from is available on a single list (overview or detail), on several
selected lists, on a single row inside a list, and on several selected rows.

• ＋ Create menu on the lists overview rows and on the list detail screen.
• Row-level menu in the list detail table, plus row multi-select with the three pills.
• To Message drafts differ by source: a list yields its title, description, up to five row
  headlines and the true total ("340 items in total"); rows yield the selected rows' headline
  values with an "…and N more" line past five.

Tests: list→draft and rows→draft text generation (including the >5 truncation and total).

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.