Switch from sending membership event for kick to using the /kick API

[aaronraimist]

I created a PR to to Synapse to align Synapse's implementation of membership events with the way that the spec says they should work matrix-org/synapse#10807. The problem with that is it will reintroduce #1961 because right now when you click the kick button in MemberInfo or use the /kick slash command Element sends a membership event for that user rather than using the dedicated API https://spec.matrix.org/unstable/client-server-api/#post_matrixclientr0roomsroomidkick

So Element needs to switch over to using the /kick API so that behavior does not come back once the Synapse PR is merged.

• Change slash command to use dedicated /kick API
• Change MemberInfo to use dedicated /kick API
• Investigate if there are any other areas where kicking or banning happen via membership events. The ban and unban buttons in MemberInfo, /ban and /unban slash commands, and the unban button in the room settings seem to all use the proper APIs.

[t3chguy]

I think this issue belongs on https://github.com/matrix-org/matrix-js-sdk but can't transfer cross-org

[novocaine]

So I think the steps to test this from an Element Web perspective are

• Check kick still works (doesn't matter where from in the UI, as we're changing the SDK implementation, not the call sites)
• Check the above at different power-levels for the kicker and the kickee matches expected behaviour

We have reason to be cautious here because breaking kick would be Bad and we don't use this API elsewhere