Use the new slot API for the modal

[jcoyne]

Closes #3102 and #3106

[jrochkind]

Oops, needed to actually work with ViewComponent 3.x, so a bug fix, yes?

[jcoyne]

@jrochkind yes, this is largely the same as #3106 but with the build passing (mostly)

[jrochkind]

Thanks! The difference between #3106 and this seems to be this one lacks additional specs.

Since the tests are being run with ViewComponent 3.x (yes?), which this func definitely didn't work with prior to this fix... the fact that we had green builds WITHOUT fixing #3102 definitely suggests we are missing some test coverage. So ideally we would add test coverage that would be red without this fix.

But if it's infeasible to provide for some reason, I guess better to have a fix that isn't covered by a test than no fix at all, I could approve anyway if it's infeasible to cover with test...

...but probably someone should at least manually confirm this really does solve #3102 and doesn't have any typos or whatever by manually exersizing func and confirming here they have?

[jcoyne]

@jrochkind I've added some tests here.

[jrochkind]

Curious why #3106 didn't pass, where it differed, but approved!