PLT-???? Prepare file upload infrastructure for Data Retention. (#7266)

* Prepare file upload infrastructure for Data Retention.

This commit prepares the file upload infrastructure for the data
retention feature that is under construction. Changes are:

* Move file management code to utils to allow access to it from jobs.

* From now on, store all file uploads in a top level folder which is the
  date of the day on which they were uploaded.

This commit is based on Harrison Healey's branch, but updated to work
with the latest master.

* Use NewAppError

api/file.go

@@ -96,7 +96,7 @@ func getFile(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := app.ReadFile(info.Path); err != nil {
+	if data, err := utils.ReadFile(info.Path); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
 	} else if err := writeFileResponse(info.Name, info.MimeType, data, w, r); err != nil {
@@ -118,7 +118,7 @@ func getFileThumbnail(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := app.ReadFile(info.ThumbnailPath); err != nil {
+	if data, err := utils.ReadFile(info.ThumbnailPath); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
 	} else if err := writeFileResponse(info.Name, THUMBNAIL_IMAGE_TYPE, data, w, r); err != nil {
@@ -140,7 +140,7 @@ func getFilePreview(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := app.ReadFile(info.PreviewPath); err != nil {
+	if data, err := utils.ReadFile(info.PreviewPath); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
 	} else if err := writeFileResponse(info.Name, PREVIEW_IMAGE_TYPE, data, w, r); err != nil {
@@ -190,7 +190,7 @@ func getPublicFile(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := app.ReadFile(info.Path); err != nil {
+	if data, err := utils.ReadFile(info.Path); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
 	} else if err := writeFileResponse(info.Name, info.MimeType, data, w, r); err != nil {
@@ -285,7 +285,7 @@ func getPublicFileOld(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := app.ReadFile(info.Path); err != nil {
+	if data, err := utils.ReadFile(info.Path); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
 	} else if err := writeFileResponse(info.Name, info.MimeType, data, w, r); err != nil {