+ submodel on top-level models

Hey team Speckle :cool_spockle:,

It’s nice that there is a + submodel button on bottom level models in the web app.

But with this button only one submodel can be added to a certain model, because after one has been added, that button disappears:

It would be handy to keep that + submodel button after the first submodel has been created.

:smiling_hearts_spockle: