Implement message history

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Currently, messages are just held in memory. Add a history (possibly with iCloud integration?)

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Work started. Storage side part is done. Now rewriting ChatViewController to make use of it.

For now, outgoing messages and incoming will be written immediately but the code will be there to pull both new and old messages directly from the message storage.

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Now using XEP-136 for this. While the XMPPFrameworks XEP-136 only supports local archiving it is better than nothing and server-side storage might work in the future without further additions. Also dropping the NSWebView in favor of a custom NSTableView that directly gets it's content from the history store.