Convert list of groups and mailings in CiviMail to be loaded dynamically

For sites that have either a large number of previous mailings or a large number of groups or both, may experience slowness on loading the new mailing screen. This is because we load in an array of mailings and groups into the DOM which then means it has to processes it all in the angular screen. An option AUG has been running with and experienced a performance improvement is to use API requests so that the list loads on demand rather than pre loading it all.