Moving slack importer into a service (#14860)

* Moving slack importer into a service

* Adding missed license headers

* Fixing tests

* Adding license header

app/app_iface.go

@@ -7,7 +7,6 @@
 package app
 
 import (
-	"archive/zip"
 	"bytes"
 	"context"
 	"crypto/ecdsa"
@@ -905,12 +904,7 @@ type AppIface interface {
 	SetTeamIconFromFile(team *model.Team, file io.Reader) *model.AppError
 	SetTeamIconFromMultiPartFile(teamId string, file multipart.File) *model.AppError
 	SetUserAgent(s string)
-	SlackAddBotUser(teamId string, log *bytes.Buffer) *model.User
-	SlackAddChannels(teamId string, slackchannels []SlackChannel, posts map[string][]SlackPost, users map[string]*model.User, uploads map[string]*zip.File, botUser *model.User, importerLog *bytes.Buffer) map[string]*model.Channel
-	SlackAddPosts(teamId string, channel *model.Channel, posts []SlackPost, users map[string]*model.User, uploads map[string]*zip.File, botUser *model.User)
-	SlackAddUsers(teamId string, slackusers []SlackUser, importerLog *bytes.Buffer) map[string]*model.User
 	SlackImport(fileData multipart.File, fileSize int64, teamID string) (*model.AppError, *bytes.Buffer)
-	SlackUploadFile(slackPostFile *SlackFile, uploads map[string]*zip.File, teamId string, channelId string, userId string, slackTimestamp string) (*model.FileInfo, bool)
 	SoftDeleteTeam(teamId string) *model.AppError
 	Srv() *Server
 	SubmitInteractiveDialog(request model.SubmitDialogRequest) (*model.SubmitDialogResponse, *model.AppError)