image thumbnails now scale appropriately so there is no whitespace, also generalized some thumbnail loading code

web/react/utils/constants.jsx

@@ -52,6 +52,8 @@ module.exports = {
   MAX_DISPLAY_FILES: 5,
   MAX_UPLOAD_FILES: 5,
   MAX_FILE_SIZE: 50000000, // 50 MB
+  THUMBNAIL_WIDTH: 128,
+  THUMBNAIL_HEIGHT: 100,
   DEFAULT_CHANNEL: 'town-square',
   OFFTOPIC_CHANNEL: 'off-topic',
   POST_CHUNK_SIZE: 60,