Implement MSC3981 in matrix-react-sdk

[Johennes]

Implement MSC3981 in matrix-react-sdk.

We construct a call to the relations API in matrix-js-sdk/src/client.ts, line 7895, in fetchRelations():

let templatedUrl = "/rooms/$roomId/relations/$eventId";
...
const path = utils.encodeUri(templatedUrl + "?" + queryString, {

fetchRelations is called from getThreadTimeline in client.ts line 5702. This is where we pass in the parameter that gets converted into queryParams. There are 4 calls in here e.g.:

const resNewer: IRelationsResponse = await this.fetchRelations(
    timelineSet.room.roomId,
    thread.id,
    THREAD_RELATION_TYPE.name,
    null,
    { dir: Direction.Forward, from: res.end },
);

We also call fetchRelations in paginateEventTimeline in client.ts line 6119, which is also relevant.

getLatestTimeline might also need changing.

In each of these calls we should pass the org.matrix.msc3981.recurse=true param (later it will be recurse=true).

What we need to do:

• Check whether the server has this feature enabled
• If so, pass this flag to the relations API during thread fetches, and don't do the whole thread.processEvent -> fetchEditsWhereNeeded stuff - instead it's much simpler - just one call to fetchRelations. Also remove this processEvent calls from thread.addEvent.