Visualize images generated in training phase

[disktnk]

This issue is for tracking the implements for Image API, which helps users to show images generated in training phase. I split the feature to several PRs.

• Server side API design: add basic logic of ImageReport extension #83
• add base view on frontend: add basic logic of ImageReport extension #83
• return error on view, ex) not install PIL, not have images or any other
• cache PNG data to DB: add data table for image API #119
• polling to call GET API to automatically update
• lazy load to image
• upload images to show already existed
• more effective UI design
• place a link to access the images page
• user guide
• examples

[disktnk]

return error on view, ex) not install PIL, not have images or any other

the timing to save image is in training process, and warning log is output on training log when Pillow is not installed. so server does not have to return error.

[disktnk]

upload images to show already existed

need POST API to upload binary, will deal it with another issue.