Fix some Smart Linking quirks

[acicovic]

Describe the bug

Smart Linking has some quirks. Specifically:

Quirk 1

1. If we ask for Smart Link suggestions and press Reject for all of them, we finish with an empty left-hand list the dialog:

2. In addition, if under that state we press Accept, the suggested Smart Link gets added to the content.

Quirk 2

Let's say that we generate 2 links for a Block. We accept the first link and we reject the second. Finally, we press Remove for the link we've just accepted. We'll get "Successfully added 1 smart links." notifications.

Quirk 3

Not really a quirk, but something which can look like one. When we've only got 1 suggestion, the Previous and Next buttons are still being displayed, and they aren't visibly disabled (although they don't work, styling remains the same).

If there is no way to have a visibly disabled state, I think we could hide them when only 1 item is in the list.