The new integrations popup was previously hard-coded into the page,

hidden by default and then wrapped in the JavaScript view, much like the

configuration list until a recent change. That information has since

been added to the list of data passed to the view, meaning we're sending

it over the wire twice. This isn't efficient and isn't worth doing,

particularly if the popup's never going to be used.
This change updates the popup rendering to happen dynamically in the

JavaScript view the first time it's clicked. This is done in a way that

minimizes render time while also allowing subclasses to override the

display.
The Add Integration button has also received an icon. This uses the same

look (a white plus symbol on a green circle) that the Add button for

condition fields use, which we'll be able to use in more places

throughout our config form UIs.

Tested that the popup displayed properly and that integrations could be

added.
Unit tests pass.