Fix integration config forms to show local site when fieldsets exist.

Review Request #1080 — Created Feb. 16, 2018 and updated — Latest diff uploaded

Review Board
All integration configs for Review Board automatically include a local
site field. In the case where there were no fieldsets, all the fields
would show up fine, but if the form tried to group things, the local
site field would disappear. This is because the djblets form was
defining the basic info fieldset with only the "name" and "enable"

I've fixed the Review Board form to properly override the basic info
fieldset, so that local site shows up everywhere.

Testing done:
Loaded some integration config forms that previously exhibited a missing
"Local Site" field. Saw the field appear.

Reviewed at