Read only and/or snapshot sharing

[cben]

In some cases it's desirable to share a document without giving write access. How to implement this on top of firebase security rules? The straightforward way is adding an edit_key child and requiring == to key users supplies. This would mean that original ?doc=foo url becomes read-only, and ?doc=foo&edit_key=bar is needed to edit. Avoiding this — letting original writable URL stay writable — probably requires moving the firepad history to a new place (which might break concurrent/offline edits at the moment it's done).

• Creating completely read-only pages is somewhat contrary to my goals for Mathdown; ideally I'd want main text to be read-only but allow comments. My current plan for comments (#41) is simply part of the text with special convention to indicate author — this would be tricky to support.
  • It might be easy to support comments outside the main text via Firepad rich text support. I don't really want to go this way though.
  • I think I'll settle for append-only comments. Those would be left outside the main document model, similar to the planned from->append fallback for users without javascript (#7).
• A related feature would be sharing a specific snapshot frozen in time (like pastebin sites).

[cben]

An alternative access model is restricting editing (or all access in the case of #92) to an ACL of specific logged-in users (once we have those — #50). I'm less inclined to support this but once a user visits a "priviledged" link with edit key, the account should "bookmark" this link which essentially gives this user access.

[Why do I prefer capability links to user identity? To some degree this is just gut feeling, but (1) same access controls apply to users that haven't created an account — or have but are not logged in right now (2) access can be delegated, which is a feature ]