Lazy load sidebar's spools

[mdziekon]

Requirements:

• Sidebar's spools are lazy loaded, instead of fetched on Plugin initialization
  • Current spool should either be lazy loaded as well (with visible spinner) or fetched separately to speed up the query
  • Selectable spools (for the selector modal) should be lazy loaded on modal display
• Spinner should be presented when loading spools
• (optional) Lazy loading is a toggleable feature

Reasoning:

• This should help low powered devices to not be resource starved on OctoPrint's page load