Add initial invitation balance to `MemberInvited` event

[Lezek123]

Add initial invitation balance to MemberInvited event to avoid having to hardcode it in the query node: https://github.com/Joystream/joystream/blob/master/query-node/mappings/src/membership.ts#L74

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[bedeho]

Done in #4646