PLT-5366, PLT-5364, PLT-5363: Bulk Import Part 1. (#5204)

This commit provides the first part of the bulk import system.

The CLI command is provided, complete with validation & apply modes. All
the basic properties of Teams and Channels can be imported. Users &
Posts will follow separately in a future commit.

app/slackimport.go

@@ -484,7 +484,7 @@ func SlackAddChannels(teamId string, slackchannels []SlackChannel, posts map[str
 
 		if mChannel == nil {
 			// Haven't found an existing channel to merge with. Try importing it as a new one.
-			mChannel = ImportChannel(&newChannel)
+			mChannel = OldImportChannel(&newChannel)
 			if mChannel == nil {
 				l4g.Warn(utils.T("api.slackimport.slack_add_channels.import_failed.warn"), newChannel.DisplayName)
 				log.WriteString(utils.T("api.slackimport.slack_add_channels.import_failed", map[string]interface{}{"DisplayName": newChannel.DisplayName}))