Work around crazy emails with non-base64 encoded attachments - #283

GMail IMAP push will get in a weird state if it encounters an email
which is encoded in this manner. The connection will be reset after a
long timeout after encountering this binary crap.
I suspect an earlier version of GMail server software, or some non-IMAP
import allows or allowed this in at some point, and yet it is not
possible to upload it back verbatim.
For my purposes, it suffices to skip these rare species but in general
the email parts should be re-encoded as base64, or the problematic
invalidly encoded attachments should be removed.
The illegally encoded binary emails are detected here by looking for the
PNG magic string with bytes that SHOULD NOT appear in any legitimate
email data.
A screenshot of one of the affected emails is at
https://imgur.com/a/i09Xa.

This commit tests all base64-encoded parts of emails to push.
This generalizes the previous ad hoc method which only worked for
detecting the PNG magic header anywhere in the message.
If a email parts/payloads claim to be base64 encoded but contain
invalid characters outside the base64 alphabet, the email is never pushed
to server. The erroneous emails are NOT sent to quarantine since they
are assumed/observed to be both rare and can be detected client
side (and notably CANNOT be detected server-side as such because they
break the protocol contract).