User-friendly cloning of room history to different homeserver

**Is your suggestion related to a problem? Please describe.**

I just moved off matrix.org to another homeserver at chat.feneas.org . I have a room I started on matrix.org that I'd like to 'move' to the new homeserver, to take some load off matrix.org, so I set a new room name ( #voice:feneas.org). But then I got told that the history of the room was still only available on matrix.org, and that to get a complete copy of the history on the new homeserver, I'd need to manually scroll up through the entire history, using an app logged into the new homeserver. The #voice room has only existed for one year, and is only lightly used, but when I did this manual scrolling to clone the history with Riot-web, it took me more than 15 minutes of continuous scrolling.

**Describe the solution you'd like**

I'd like to have a button that clones the entire history of a room to the homeserver I'm logged into

**Describe alternatives you've considered**

. Or maybe a tickbox in the area of 'Room Settings' where users set a new alias for the room on their homeserver;

> Copy full room history to your homeserver: Y/N?

**Additional context**

Since most users experiment with Matrix for the first time by creating an account on matrix.org, the vast majority of Matrix rooms are created on matrix.org. Because matrix.org is the only homeserver with the full history of most rooms, it's forced to serve as a de facto central history database for the whole Matrix federation. With a simple, user-friendly way to 'move' rooms, including a copy of their full chat history, the federation would become much more decentralized, and reduce the server load on matrix.org.

No files, tests, or entry points are named. Start by mapping how Matrix federation currently makes room history available across homeservers and where Riot-web initiates history loading; done requires a defined, user-facing way to copy complete room history to the logged-in homeserver.

Written by the indexing model from the issue text.