CIF-1609 - Lazy Loading for PLP

[cjelger]

This PR adds lazy loading to the category and search pages vis the productcollection component. The pages are still rendered on the server, and are requested with a "load more" button and then appended to the display by some JS code.

See the screenshots: configuring the component to lazy-load adds a "Load more" button, clicking it displays a loading spinner until the next page is loaded.

The css changes are in adobe/aem-cif-guides-venia#61

How Has This Been Tested?

Extended unit tests and tested manually.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes and the overall coverage did not decrease.
• All unit tests pass on CircleCi.
• I ran all tests locally and they pass.

[codecov]

Codecov Report

Merging #406 into master will increase coverage by 0.05%.
The diff coverage is 93.93%.

@@             Coverage Diff              @@
##             master     #406      +/-   ##
============================================
+ Coverage     86.06%   86.11%   +0.05%     
- Complexity      947      950       +3     
============================================
  Files           188      188              
  Lines          4886     4918      +32     
  Branches        696      697       +1     
============================================
+ Hits           4205     4235      +30     
- Misses          539      541       +2     
  Partials        142      142              

Flag	Coverage Δ	Complexity Δ
#integration	67.29% <80.00%> (+0.01%)	691.00 <1.00> (+2.00)
#jest	81.55% <ø> (ø)	0.00 <ø> (ø)
#karma	94.24% <92.85%> (-0.08%)	0.00 <0.00> (ø)
#unittests	86.29% <100.00%> (+0.05%)	919.00 <2.00> (+4.00)

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Impacted Files	Coverage Δ	Complexity Δ
...oductcollection/clientlibs/js/productcollection.js	91.56% <92.85%> (+0.65%)	0.00 <0.00> (ø)
...ls/v1/productcollection/ProductCollectionImpl.java	96.29% <100.00%> (+0.29%)	14.00 <1.00> (+1.00)
.../components/internal/services/UrlProviderImpl.java	86.66% <100.00%> (+0.22%)	22.00 <0.00> (+1.00)
...ommerce/core/search/internal/models/PagerImpl.java	94.59% <100.00%> (+0.15%)	20.00 <1.00> (+1.00)

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