Messages: Share — copy permalink and Get embed code #21

Part of #17 — Message composer & feed parity gaps

Every web message has a permanent page at /user/<username>/status/<id>, and a Share
button offering Copy link and Get embed code (an HTML snippet for pasting into a blog —
viewers need no account for a public message).

Only public messages can be linked or embedded for other people; a private message's link and
embed work only for its owner while signed in.

• Add a Share action to the message action bar: copy link, get embed code, and the Android system
  share sheet (which the web has no equivalent of but users will expect).
• Suppress/clearly mark the options on a private message.

Tests: permalink construction from username + id; embed snippet shape; private message
behaviour.

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