Turn off autocomplete for username/password fields in E-Mail Settings

When the username and password fields are set to blank and saved,
the fields are automatically filled by Chrome with the previous values,
indicating that the blank values are not saved successfully even though they
are. Turning off autocomplete for these fields avoids such confusion.

Testing done:
Manually tested updating username/password with both empty and
non-empty values.

Unit tests pass.

Fixes bug 2380.
Reviewed at https://reviews.reviewboard.org/r/4579/