Fix missing variable declarations in auth backends.

The auth backends were attempting to decode UTF-8 username strings into
byte strings before operating on them, in order to convert them to a
format expected by the libraries providing authentication. If a byte
string is passed in, though, the variables are never declared, causing
errors.

This fixes this case, adding the missing variable declarations.

Reviewed at https://reviews.reviewboard.org/r/6672/
Fixed a very small typo in the Web API documentation.

In the documentation for Review Request Resource, the description of `approved`
should be "a review request is approved if it has at least one Ship It! and
*no* open issues."

Reviewed at https://reviews.reviewboard.org/r/6673/
Merge branch 'release-2.0.x'