Fix parse_username to handle Unicode

This patch fixes some general UTF-8 encoding issues, including:

(1) Passing connect_args parameters to SQLAlchemy so that the
database returns UTF-8

(2) Modifying parse_username; parse_username was throwing a
UnicodeDecodeError because it is unable to handle input of type
<unicode>, only <str>. This change modifies validate_cohort so that
it does not call parse_username, and also corrects an error in
parse_username where the first *byte* (rather than *character*) of
a UTF-8 string was capitalized.

Addresses bug 63836
Change-Id: Ie889a0241c8fb61522b713f5e5590c99635e8917