What's new: the changelog window

New in 0.3, module changelog, off by default. Players with Keystone's client half see what changed on the server once after each update, drawn by Keystone's own light renderer: headings, paragraphs, lists, bold, italic and links - nothing that can run.

• source: an https page (HTML or Markdown) with the changelog. {lang} in it becomes the player's language (en, cs …), from languages (default en, cs); a player in any other language gets the first one.
• text: the changelog written right here in Markdown (# heading, - item, **bold**, [link](https://…)), used when source is empty.
• title: the window's title; empty takes the page's first heading.
• show: once-per-update (the first join after the page changed, default), every-join, or off (only /changelog).
• delay: how long after joining the window opens (4 s), so the world loads first.
• more: an https address for the "Full changelog" button; empty shows the source address, or no button.
• allow-local: let source be on this machine or network (owner only). Otherwise the page is fetched over https only, never from inside the network and without following redirects.

/changelog opens the window any time; without the client half it answers in chat. GET /v1/changelog (scope players.read) returns what players are shown.