Make draft RegionCommentBlock movable and resizable.
Currently, a RegionCommentBlock is not movable or resizable, even if it is a draft. This change makes draft RegionCommentBlocks movable or resizable by drag-and-drop'ing left-top or right bottom corner of them. Users finish resizing by mouse-up action, and the draft RegionCommentBlock's new bound is saved on server. RegionCommentBlockView handles mouseup and mousedown, and simulate click event if mouseup happened w/o any drag. Click on RegionCommentBlockView shows the comment inside, and shows an editor to change it, if any comment in the block is a draft.
I made this RR to combine two RRs:

https://reviews.reviewboard.org/r/6886/

https://reviews.reviewboard.org/r/6919/

and fix click/drag-and-drop bugs in them.

Passed all tests in "/js-test/".
Browser test done:

  1. Go to file attachment review page, for example, http://localhost:8080/r/7/file/4/

  2. Make draft comment

  3. Try moving

  4. Try resizing

  5. Refresh the page

  6. See if the bounds updated
Edge cases tested:

  1. Open file attachment review page, make resize/move "once" and refresh. => Fixed and working well.

  2. Try click-and-drag non-draft commentBlocks. => Non-drafy commentBlcoks are not movable. Working as expected.