[local-sync] Be more permissive with content transfer encodings (CTEs)
[local-sync] Be more permissive with content transfer encodings (CTEs)

Summary:
Rather than having a strict model where we don't decode the message if we
don't specifically recognize the CTE, treat any CTEs we don't recognize as
having no encoding. There are several CTE strings that could mean this (e.g.
7bit, 7BITS, 8-bit, binary, NONE, utf8), and we don't want to check for them
all. Additionally, if there is a CTE we don't support, the user will likely
see rendering issues and contact support. This will allow us to obtain more
concrete information about these messages.

Test Plan: manual

Reviewers: spang

Reviewed By: spang

Differential Revision: https://phab.nylas.com/D3748

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