Add mapped Mattermost users as attendees in CreateEvent API method

Summary

The api.CreateEvent() method accepts a string of Mattermost UserID values, which will be used to check if the user is mapped to an office account, in the plugin store.

Currently, the method checks if a user is mapped, and sends that user a Bot DM with information to connect through the plugin if they are not mapped. The task here is to add the case where a Mattemermost UserID is mapped and their mapped user information needs to be added to the remote.Event struct so they receive the invite.

Unit tests can be written following the mocking methods used here: https://github.com/mattermost/mattermost-plugin-mscalendar/blob/master/server/api/availability_test.go

Ticket Link

https://mattermost.atlassian.net/browse/MM-21349