Load the thread list using server-side sorting and pagination

[daniellekirkwood]

We currently load the entire thread list without pagination and sort it on the client so that we can sort by last-replied-to.

Add support for that sort to the server query and load the list using pagination.

• Server side: Implement MSC3856, a threads list API matrix-org/synapse#13921
• Web: Load the thread list using server-side sorting and pagination element-web#21877
• iOS: Load the thread list using server-side sorting and pagination element-ios#6059
• Android: Load the thread list using server-side sorting and pagination element-android#5819

[clokep]

MSC3856: Threads List API was written to solve this issue.