#19550 Enhancement: Refactor rack elevations template for lazy loading /dcim/rack-elevations/

[tbotnz]

Closes: #19550

The existing rack elevations template does not scale well, for e.g if the user has the pagination set 1000, the rack elevation template will try and make a rack http call for every rack when the page loads (#19550)

this PR does the following

• introduce lazy loading so that racks are only loaded once they intersect with the viewport
• spinner added to show while racks are being lazy loaded