Searching conversations by multiple inboxes
We have a Front integration that involves fetching conversations within a timeframe from selected inboxes, at the moment we paginate through the conversation search endpoint for each inbox ID. However, to ensure that we fetch all the conversations in that timeframe it would be much easier for us if the inbox filter was treated as one of the `OR` filters (from, to, cc, bcc) so that we could filter `inbox:A inbox:B after:<date> before:<date>`
