Coeditors don't see my changes in the realtime wiki editor after a brief connection loss

Summary of the fix (realtime wiki editor):

• When the editor reconnects and the realtime session it was using has been removed in the meantime (because the user was editing alone), it now starts a new realtime session instead of reusing its previous synchronization state.
• The new session starts from the saved content, which is what the users joining later start from. The content of the editor, including the unsaved changes, is added as a change on top of it, so it reaches everyone who joins afterwards.
• If someone joined and modified the new session before the editor rejoined it, the changes of both users are merged instead of one of them being lost.
• A document that was never saved is considered empty.

The document version and the auto-save are not affected, so the merge conflict detection on save works as before.

Verified locally with RealtimeWikiEditorIT#toggleRealtimeWithSelf (forced into the state that fails on CI, then 5 unforced runs each on Firefox and Chrome) and with a temporary scenario where a second user modifies the new session before the first one rejoins it. The realtime WYSIWYG editor has a milder variant of this problem, which will be handled separately.