[MM-28422] Enable processing of import files through API (#16062)

* Implement unzip function

* Implement FileSize method

* Implement path rewriting for bulk import

* Small improvements

* Add ImportSettings to config

* Implement ListImports API endpoint

* Enable uploading import files

* Implement import process job

* Add missing license headers

* Address reviews

* Make path sanitization a bit smarter

* Clean path before calculating Dir

* [MM-30008] Add mmctl support for file imports (#16301)

* Add mmctl support for import files

* Improve test

* Remove unnecessary handlers

* Use th.TestForSystemAdminAndLocal

* Make nouser id a constant

api4/import.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package api4
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/v5/model"
+)
+
+func (api *API) InitImport() {
+	api.BaseRoutes.Imports.Handle("", api.ApiSessionRequired(listImports)).Methods("GET")
+}
+
+func listImports(c *Context, w http.ResponseWriter, r *http.Request) {
+	if !c.IsSystemAdmin() {
+		c.SetPermissionError(model.PERMISSION_MANAGE_SYSTEM)
+		return
+	}
+
+	imports, appErr := c.App.ListImports()
+	if appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	data, err := json.Marshal(imports)
+	if err != nil {
+		c.Err = model.NewAppError("listImports", "app.import.marshal.app_error", nil, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	w.Write(data)
+}