With the advent of high-resolution screens for mobile devices and laptops (and

more and more for desktops), it's common for UIs and the icons/graphics

displayed therein to be shown at a pixel ratio where the browser uses multiple

physical pixels (either 4 or 9) for a single "virtual" pixel.
This means that most screenshot tools will produce images that show super huge

inside Review Board.
This change adds a new drop-down menu to the image review UI to select a zoom

level. We offer 33% (good for 3x ratios), 50% (for 2x), 100%, and 200% (which

is good for getting a close look at alignment and such). In the case where the

filename of the image being reviewed includes the magic text "@2x" or "@3x",

the appropriate zoom level will be pre-selected.

Checked each zoom level with:

A plain image file attachment.
An image attachment with multiple revisions (no diff)
An image attachment with multiple revisions for each diff mode.


Tested the pre-selection of the right zoom level for both plain image

  attachments and each diff mode.
Tested loading existing comments at various zoom levels.
Tested creating comments with different review modes and zoom levels.

  Verified that the comment regions were assigned correctly.
Tested moving/resizing comment regions at different zoom levels and verified

  the results.
Changed zoom levels and verified that comment regions were resized along with

  the images.
Checked functionality of the drop-down menu for each of the different display

  modes in mobile sizes.
Ran js-tests.
Ran unit tests.