Added a spinner that indicates when an image summary at a certain step is loading.
Added a spinner that indicates when an image summary at a certain step is loading.

This avoids flickering when the user scrolls through steps of an image summary, allowing the user to readily compare images across steps.
Change: 146180073

qiuminxu/tensorboard,francoisluus/tensorboard-supervise,ioeric/tensorboard,agrubb/tensorboard,francoisluus/tensorboard-supervise,ioeric/tensorboard,qiuminxu/tensorboard,agrubb/tensorboard,ioeric/tensorboard,shakedel/tensorboard,qiuminxu/tensorboard,shakedel/tensorboard,tensorflow/tensorboard,francoisluus/tensorboard-supervise,qiuminxu/tensorboard,agrubb/tensorboard,qiuminxu/tensorboard,francoisluus/tensorboard-supervise,agrubb/tensorboard,agrubb/tensorboard,agrubb/tensorboard,shakedel/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,francoisluus/tensorboard-supervise,tensorflow/tensorboard,ioeric/tensorboard,francoisluus/tensorboard-supervise,tensorflow/tensorboard,shakedel/tensorboard,shakedel/tensorboard,shakedel/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,ioeric/tensorboard,ioeric/tensorboard,qiuminxu/tensorboard