Closes #5858: Implement a quick-add UI widget for related objects

[jeremystretch]

Closes: #5858

• Extend the APISelect widget to support a "quick add" button, which launches an embedded modal
• Extend ObjectEditView to support a new "quick add" workflow (designated by the _quickadd=True URL query parameter)
• Adapt initReslug() to accommodate multiple "reslug" buttons on the page
• Add quick_add=True to various form fields as appropriate

[jeremystretch]

There's still a bug involving the quick selector widget when displayed within the quick add form. For instance, opening the power feed form, clicking the "quick add" button for power panel, and then clicking the selector button next to the site field will close the model. This is because the selector button toggles the currently open modal, closing it.

Unfortunately, Bootstrap doesn't provide a mechanism for explicitly displaying a modal via an HTML data attribute. We can introduce an event listener for this, however that seems to complicate the logic for dismissing modals as well. Need to dig into this a bit more; not sure it's worth holding up the whole PR for it.

[arthanson]

• slug fields don't auto-populate if there is a slug field also on the launching form.
• regenerate slug (spinner to the right of slug field) regenerates based on original form, not modal form name.
• It would also be nice to have the quick-added field auto-selected when you create it in the form that you launched it from - that is what I would expect to happen. Here you quick add it, then need to select it.

[arthanson]

Maybe add a comment do not set it if the folllow-on form has any quick add fields.