Don't calculate hex digest for non-ascii email addresses.

Non-ascii chars shouldn't be validating email addresses but just in case
email addresses come from other sources, let's catch the exception.

Fixes bug 2812.
No need to use the Toolkit's wrapper around md5 functions.

Since Python 2.5 the md5 module is deprecated in favor of hashlib.
Removed unused import.
Fixed failing import.
Renamed variable to keep proper semantics.
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Remove unnecessary check in handle_form() view in pootle_notification app