Convert integration tests to Cypress (#14174)

This completes one step towards removing Enzyme.
The complete list of all tests is at #13657.

* Run each test in isolation

* Convert `EditUserModal.integ.spec.js` to Cypress

* Convert `PeopleListingApp.integ.spec.js` to Cypress

* Convert `GroupDetailApp.integ.spec.js` to Cypress

* Delete `GroupsListingApp.integ.spec.js`

It was already covered in Cypress.

* Delete `NewUserModal.integ.spec.js`

It was already covered in Cypress.

* Convert `UserActivationModal.integ.spec.js`

* Convert `UserPasswordResetModal.integ.spec.js`

* Convert `UserSettings.integ.spec.js`

* Remove integration tests

* Remove integration tests from CI

* Fix linter errors

* Delete `AuditTable.integ.spec.jsx`

* Clean up `people.cy.spec.js`

* Update assertion that makes sure XHR didn't happen

* Remove redundant command from PR template

Note: `yarn-lint` already runs `yarn flow`.
https://github.com/metabase/metabase/pull/14174#discussion_r549790198

* Remove explicit mention of timezones in `developers-guide.md`