synapse secretly uses body of `/join` request as the join event content

This issue has been migrated from [#8763](https://github.com/matrix-org/synapse/issues/8763).

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This is totally unspecced, but it turns out that synapse uses the body of a [`/_matrix/client/r0/join/{roomIdOrAlias}`](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-join-roomidoralias) request as the content of the join event.

Apart from being inconsistent with the spec, it also means that there is nowhere to put other options such as `server_name` (see https://github.com/matrix-org/matrix-doc/issues/2864).

(If there's a need for such a thing, it should be in a `content` key)