Fix failure to import same Slack archive to 2 teams. (#4650)

This fixes the issue where you can't import 2 Slack exports with any
users who's emails appear in both exports onto different teams of the
same server.

Fixes #4126

api/slackimport.go

@@ -162,7 +162,11 @@ func SlackAddUsers(teamId string, slackusers []SlackUser, log *bytes.Buffer) map
 		if result := <-Srv.Store.User().GetByEmail(email); result.Err == nil {
 			existingUser := result.Data.(*model.User)
 			addedUsers[sUser.Id] = existingUser
-			log.WriteString(utils.T("api.slackimport.slack_add_users.merge_existing", map[string]interface{}{"Email": existingUser.Email, "Username": existingUser.Username}))
+			if err := JoinUserToTeam(team, addedUsers[sUser.Id]); err != nil {
+				log.WriteString(utils.T("api.slackimport.slack_add_users.merge_existing_failed", map[string]interface{}{"Email": existingUser.Email, "Username": existingUser.Username}))
+			} else {
+				log.WriteString(utils.T("api.slackimport.slack_add_users.merge_existing", map[string]interface{}{"Email": existingUser.Email, "Username": existingUser.Username}))
+			}
 			continue
 		}