Improve loading interstitial for filter blocks and All Products Blocks

[nerrad]

The loading interstitial we currently have for the All Products and filter blocks is used to indicate when something is being retrieved in the background. However, I think this is something we could do better. For reference here's a gif of the current behaviour:

Some rough ideas:

• Leave current view in place and only swap out when we have the new results.
• The above would require an alternative loading indicator - would it be better to just have a pulsing dot somewhere in the view? Do we disable filters/paging while results are loading?
• I think we'll want some subtle transition animation when the view changes on new results.

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[nerrad]

This is a duplicate of #1187