rbintegrations was no longer working with Review Board 5, for a couple

reasons.
First, it used deprecated imports that Review Board 5 no longer

provides (as of this change, though that removal is being reverted

for now). These were attempting to import

reviewboard.integrations.Integration. We now use the

reviewboard.integrations.base.Integration import instead.
Second, we had calls in CircleCI to Django's deprecated

urlquote_plus() (which now just wraps quote_plus()) that were

failing due to passing in a Unicode string when a byte string was

expected. We now pass in a byte string.
In the process of fixing the urlquote_plus() issue, I found we were

convert a None value into the string 'None'. This seems to have been

a mistake, except the unit tests were matching that. I suspect this was

more a result of copy/paste than deliberate design.
This has been updated to just convert to an empty string and check

against that, which isn't really a whole lot better, but it's at least

more likely to trigger some meaningful error on CircleCI. A task has

been added to track this.

All unit tests pass.