test: percy visual testing (#1053)

* test: percy visual testing init

* Try PR checks with custom command

* Only run percy visual tests for PWA on merge for now

platform/viewer/cypress/integration/common/OHIFStudyViewer.spec.js

@@ -12,6 +12,8 @@ describe('OHIF Study Viewer Page', () => {
 
   it('checks if series thumbnails are being displayed', () => {
     cy.screenshot();
+    cy.percySnapshot();
+
     cy.get('.ThumbnailEntryContainer')
       .its('length')
       .should('be.gt', 1);