API support for reactions v1

[manuroe]

From riot-web created by jryans: element-hq/element-web#9485

We'll need API support for aggregations (to support message editing and reactions). This will likely be broken down into sub-tasks in React and JS SDKs once the shape of the API is known in more detail.

For the moment, matrix-org/matrix-spec-proposals#1849 gives some idea of the direction here.

• parses and stores aggregated data
• parses relates_to events
• sends reactions
• provides reactions based on event.unsigned.relations.annotations (data got after an initial /sync)
• provide updated reactions
  • dedicated store for aggregated reactions
  • have a mechanism to notify updated reactions
  • store aggregated reactions only for events in the timeline we have (other may be not accurate)
  • keep event id of reactions made by the user (for unreaction and for displaying them differently in the UI)
  • keep it even if the event is not (or no more) in our the timelines
• unreact
• handle incoming unreaction

Moved to specific issues:

• manage reactions for timelines opened on permalink -> Reaction: Support in non live timeline #2451
  • update reactions on reactions update
• unreact when the reaction event id is not known (?)
• reactions in e2e rooms -> Support clear reactions in e2e rooms #2448
• call reaction success block only when reaction is stored -> Aggregation API: Local echo for reactions #2453
• Interface-driven reaction counts -> Aggregation API: Local echo for reactions #2453

Performance: ->

• filter out reaction events when paginating -> Aggregation API v2: Performance improvement #2454
• happy with lot of gappy syncs (?) ->