Declare support for Matrix 1.4

Some of these are good first issues

Complete changelog v1.4

Client-Server API

Backwards Compatible Changes

• Add a .m.rule.room.server_acl push rule to match m.room.server_acl events, as per MSC3786.
• Add Cross-Origin-Resource-Policy (CORP) headers to media repository, as per MSC3828.
• Copy a room's type when upgrading it, as per MSC3818.
  • We copy the whole create event and remove unwanted fields
• Add room_types filter and room_type response to /publicRooms, as per MSC3827.
  • room_types exists, but is unused
• Add m.replace relations (event edits), as per MSC2676.
  • Looks like we don't have server-side aggregation atm
• Add m.read.private receipts, as per MSC2285.
• Make m.fully_read optional on /read_markers, as per MSC2285.
• Allow m.fully_read markers to be set from /receipts, as per MSC2285.
• Add threading via m.thread relations, as per MSC3440, MSC3816, MSC3856, and MSC3715.
  • While partly working, there are still missing endpoints, e.g. GET /rooms/{roomId}/threads
• Add per-thread notifications and read receipts, as per MSC3771 and MSC3773.
• Add thread_id to the /receipt endpoint, as per MSC3771.

Spec Clarifications (NOTE: Non-actionable things removed)

• Clarify that the "device_id", "user_id" and "access_token" fields are required in the response body of POST /_matrix/client/v3/login.
  • We already return those fields

Server-Server API

Backwards Compatible Changes

• Add per-thread notifications and read receipts, as per MSC3771 and MSC3773.
  • Missing thread_id in type ReceiptTS struct (which should be renamed, since it's not TS only afterwards)

Application Service API

Breaking Changes

• Replace homeserver authorization approach with an Authorization header instead of access_token when talking to the application service, as per MSC2832.