Improve the loading state

[ndiego]

Prior to version 0.3.0, the button that launches the Block Pattern Explorer (BPE) was disabled while pattern category types are fetched from block-pattern-explorer/v1/pattern-category-types. Category types are an enhancement unique to the BPE. On slow sites, this "wait" was noticeable and jarring. We opted to remove the disabled state on the button in 0.3.0 but a better solution is needed.

Now if the use clicks the button too quickly, the default state of the explorer leaves a lot to be desired.