In RB 4.0.x, the popup that comes up after selecting an integration's Delete

button fails to disappear after deleting the integration and after selecting

the cancel button. The issue was that in RB 3.0.x, we are using 

<input type="button"> to construct the buttons and from RB 4.0.x and

onwards we are using the <button> tags instead. I've added a condition so

that the popup can be destroyed even if the <button> tags are used.

Manual testing done with the following repo pairs:

  - Tested Djblets release-2.0.x with RB release 4.0.x

  - Tested Djblets master with RB master