Refine durable column class toggling

Summary: don't bother with margin class if column isn't visible. Ref T8151.

Test Plan:
using chrome with trackpad

• loaded home with toggled off column - noted no margin class erroneously applied
• toggled column off and on - correct
• loaded home with toggled on column - correct
• toggled column off and on - correct

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8151

Differential Revision: https://secure.phabricator.com/D12795