16439: Enables the +NEW button only when the user's view is writable.

16439: Enables the +NEW button only when the user's view is writable.

Also, makes the SidePanelButton component to not depend on a state property
set by a something rendered later, because that makes the button being
re-renderered at least twice on every location change, instead try to get the
current item id from the location url.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas@di-pentima.com.ar>