Create from…: entry points on messages (single message and multi-select) #12

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

Web behaviour: a ＋ Create ▾ menu on a single message, and — in Select mode on the
feed — To List / To Doc / To List & Doc pills in the selection bar for several messages at once.

• Add the ＋ Create menu to MessageCard / the message detail screen.
• Add a multi-select mode to the feed with a selection bar. This is a new interaction for the
  Android feed and is the larger half of this issue.
• Default columns when messages become a list: Content, Author, Posted, Links, Tags, one row
  per message.

Tests: selection state ViewModel test; menu → materialize call carries every selected id.

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