Allow scenes and avatars to be Default without copying and pasting URL

[DougReeder]

Is your feature request related to a problem? Please describe.
It's counterintuitive and undocumented that scenes and avatars can only become Default by pasting the URL into the Import Content tab.

Describe the solution you'd like
The table in the Scenes tab should have a button to make default, like the current Approve button.

Describe alternatives you've considered
Something could be done on the Import Content tab, but that would still be counterintuitive.