Update rooting descriptions in tree list

Also added implicit selection of the current root node (signaling
intent) if curator marks a new tree as non-arbitrarily rooted.
Remove outdated styles and logic for @root flags

No more unrooted or multiply-rooted trees! We're going to maintain a
single @root marker, which should move in tandem with the
ot:specifiedRoot value and the "natural" root in the Nexson hierarchy.
(This logic will be added next.)
Simplify tree-rooting logic, remove un-root tool.