Implement upload and get file endpoints for APIv4 (#5396)

* Implement POST /files endpoint for APIv4

* Implement GET /files/{file_id} endpoint for APIv4

i18n/en.json

@@ -1165,6 +1165,10 @@
     "id": "api.file.upload_file.too_large.app_error",
     "translation": "Unable to upload file. File is too large."
   },
+  {
+    "id": "api.file.upload_file.bad_parse.app_error",
+    "translation": "Unable to upload file. Header cannot be parsed."
+  },
   {
     "id": "api.file.write_file.configured.app_error",
     "translation": "File storage not configured properly. Please configure for either S3 or local server file storage."
@@ -3519,6 +3523,10 @@
     "id": "model.channel_member.is_valid.user_id.app_error",
     "translation": "Invalid user id"
   },
+  {
+    "id": "model.client.read_file.app_error",
+    "translation": "We encountered an error while reading the file"
+  },
   {
     "id": "model.client.connecting.app_error",
     "translation": "We encountered an error while connecting to the server"