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

i18n/en.json

@@ -1233,22 +1233,6 @@
     "id": "api.file.move_file.rename.app_error",
     "translation": "Unable to move file locally."
   },
-  {
-    "id": "api.file.open_file_write_stream.configured.app_error",
-    "translation": "File storage not configured properly. Please configure for either S3 or local server file storage."
-  },
-  {
-    "id": "api.file.open_file_write_stream.creating_dir.app_error",
-    "translation": "Encountered an error creating the directory for the new file"
-  },
-  {
-    "id": "api.file.open_file_write_stream.local_server.app_error",
-    "translation": "Encountered an error writing to local server storage"
-  },
-  {
-    "id": "api.file.open_file_write_stream.s3.app_error",
-    "translation": "S3 is not supported."
-  },
   {
     "id": "api.file.read_file.configured.app_error",
     "translation": "File storage not configured properly. Please configure for either S3 or local server file storage."
@@ -1261,6 +1245,30 @@
     "id": "api.file.read_file.reading_local.app_error",
     "translation": "Encountered an error reading from local server storage"
   },
+  {
+    "id": "utils.file.remove_file.local.app_error",
+    "translation": "Encountered an error removing file from local server file storage."
+  },
+  {
+    "id": "utils.file.remove_file.s3.app_error",
+    "translation": "Encountered an error removing file from S3."
+  },
+  {
+    "id": "utils.file.remove_file.configured.app_error",
+    "translation": "File storage not configured properly. Please configure for either S3 or local server file storage."
+  },
+  {
+    "id": "utils.file.remove_directory.local.app_error",
+    "translation": "Encountered an error removing directory from local server file storage."
+  },
+  {
+    "id": "utils.file.remove_directory.s3.app_error",
+    "translation": "Encountered an error removing directory from S3."
+  },
+  {
+    "id": "utils.file.remove_directory.configured.app_error",
+    "translation": "File storage not configured properly. Please configure for either S3 or local server file storage."
+  },
   {
     "id": "api.file.upload_file.bad_parse.app_error",
     "translation": "Unable to upload file. Header cannot be parsed."
@@ -5575,6 +5583,10 @@
     "id": "store.sql_file_info.save_or_update.app_error",
     "translation": "We couldn't save or update the file info"
   },
+  {
+    "id": "store.sql_file_info.permanent_delete.app_error",
+    "translation": "We couldn't permanently delete the file info"
+  },
   {
     "id": "store.sql_job.delete.app_error",
     "translation": "We couldn't delete the job"