Implement the activity dates functionality for mod_chat and output them in view.php

should pass phpunit and behat tests Create a new chat module, in `Chat sessions` settings, choose "Next chat time" in the future, "Repeat/publish session times", choose "No repeats - publish the specified time only" go back to chat view page, should see "Next chat time 21 April 2021, 8:55 pm" go to chat settings, update next chat time to a date in the past and "Repeat/publish session times", choose "At the same time every day" go back to chat view page, should see next chat time in the future go to chat settings, update next chat time to a date in the past and "Repeat/publish session times", choose "At the same time every week" go back to chat view page, should see next chat time in the future