Changes

#139: Release 4.2.0 by @Progi1984
#138: Merge master by @Progi1984
#135: Update version to 4.2.0 & Some Project Improvements by @Progi1984
#137: Fix outdated license headers by @matks
#136: Add CONTRIBUTORS file by @matks
#134: Fixed the display & behavior for "Uncheck/Check All" button by @Progi1984
#104: Change include init.php place by @arthtux
#133: Enable Release Drafter by @matks
#132: Add GitHub PR template by @matks
#131: Update readme to add multistore compatibility by @marionf
#127: Fix failure when generating sitemap with >25000 links by @DavidBoone
#107: Add all images of a product to the sitemap, instead of only the cover image by @Pixep
#106: Do not call getIsset() with getValue() by @Quetzacoalt91

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