add more options to confirmation popovers...

[ajb]

...rework the API a little bit, too.

cf #289

I didn't add the advanced positioning, since that would best be
addressed upstream in Bootstrap.

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• ❌ Whether the popover is aligned to the horizontal left, right, or center of the object.
• ✅ The class(es) of the primary button. (Default: primary)
• ✅ The class(es) of the secondary button (Default: "no class")
• ✅ Also, we should remove delete_ from some classes (like popover_delete_confirmation_message to get rid of code smells.)