Extracting email functions into a service (#14802)

* Extracting email functions into a service

* Fixing two shadowing errors

* Address PR review comments

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

cmd/mattermost/commands/user.go

@@ -623,7 +623,7 @@ func inviteUser(a *app.App, email string, team *model.Team, teamArg string) erro
 		return fmt.Errorf("Email invites are disabled.")
 	}
 
-	a.SendInviteEmails(team, "Administrator", "Mattermost CLI "+model.NewId(), invites, *a.Config().ServiceSettings.SiteURL)
+	a.Srv().EmailService.SendInviteEmails(team, "Administrator", "Mattermost CLI "+model.NewId(), invites, *a.Config().ServiceSettings.SiteURL)
 	CommandPrettyPrintln("Invites may or may not have been sent.")
 
 	auditRec := a.MakeAuditRecord("inviteUser", audit.Success)