Investigate removing "special" meaning of symbols page

[jaridmargolin]

There are several branches of logic that place a special meaning on the "Symbols Page". It is worth investigating if we can remove these branches in order to simplify internals and provide a more consistent and expected interface.

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Discussed during review of #297

mathieudutour: I'm wondering if we really need to forbid rendering into the Symbols page. There is nothing special about in Sketch except being the default location where symbols are going but they can be created in any page and you can any type of layers to the Symbols page

jaridmargolin: I more or less agree with your sentiment, but I think that is a larger question that would need to be addressed in a standalone task. The current implementation of symbols makes a few assumptions about the Symbols page.

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cc @mathieudutour

[mathieudutour]

I'm wondering if we could provide a SymbolMaster component that creates a symbol from its child and could be rendered anywhere. Additionally, we could have a SymbolInstance component with a symbolMasterName prop. We would need to make sure that the symbol master is rendered before the instance tho. But if we manage to do it, we can remove the entire makeSymbol/injectSymbols hack