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

Start with the Symbols Page handling and the symbols implementation assumptions described in this issue, then read the review discussion in #297. Identify each branch that gives the page special treatment and determine whether removing those assumptions can preserve a consistent interface; done means the scope and required changes are clearly established.

Written by the indexing model from the issue text.