Multiple invite support

[lampholder]

If I understand it correctly:

• the 'invite to this room' dialogue supports inviting multiple users
• Riot is responsible for turning each of those users into individual invite requests to the server
• if all of these requests are spammed at once, the server rate limits with a 'M_LIMIT_EXCEEDED' error

The chosen solution is to implement retries. In Riot Web, if we receive a 'M_LIMIT_EXCEEDED' response, we queue the request up to try again in 5 seconds.

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Linking for reference:

• [x] vector-im/riot-web#1713
• [ ] vector-im/riot-ios#721
• [ ] vector-im/riot-android#625

[lampholder]

@dbkr - can you confirm my understanding of the problem + solution is correct?

Additionally, solving this problem client-side could be implemented quite differently across different clients (exponential vs fixed window, retry counts, client-side rate limiting, etc). How much do we care about Riot Web and iOS/Android solving this in the same way?