Add function to idempotently set reserved margins for modules.
Add function to idempotently set reserved margins for modules.

For example, themes needs to reserve 200 px of space on the left.
Comments will hopefully benefit from this as well.

plotly/plotly.js,aburato/plotly.js,asolagmbh/plotly.js,aburato/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,iongroup/plotly.js,etpinard/plotly.js,plotly/plotly.js,iongroup/plotly.js,plotly/plotly.js,etpinard/plotly.js,iongroup/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js