GitLab

- Clarify types and distinguish between MAM query options and RSM query options.

- Remove the `fetchMessagesOnScrollUp` event handler. Instead messages will be fetched as a placeholder comes into view.

- Remove support for mam2:#extended querying
    It's too much of a headache with patchy server support.
    Instead, use RSM's `<before>some-id</before>` together with `start` to
    bracket queries for placeholders.

- Create a placeholder after restoring cached messages.
    This placeholder will automatically fetch older messages when the user scrolls up.

- Create new config option `auto_fill_history_gaps`

- Don't duplicate already existing MAM placeholders.
    Pre-existing ones could have been created from the cache.

- If we have a most recent message, query from it