Add history saving functions

However, they were used from nowhere.
Add history saving
Merge branch 'feature/history-saving'
Add timestamp getter to Image class
Add history syncing function

And also change return type of HistoryManager.getAll method.

This is preparation for the history viewer.
Add history area to MainActivity
Allow expand history area via drugging
Display local history.
Display synced history in history view.
remove redundant syncing.
Enable reload history on return from new uploaded image viewing.
Use cached thumbnail during loading it from remote

To improve UX.
Change the timing of loading local history.
Simplify history view expanding process.

Now history view size is fixed, just change height of buttons view.
Fix if-modified-since request header.

If-Modified-Since header parameter value format was worng.
Make thumbnail square which filled with the image.
Add loading image
Merge branch 'feature/history'