Investigate removing "special" meaning of symbols page
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 