More precise, controlled zoom on images

[sylvieed]

You can zoom in/out on page images but the zoom isn't very easy to use. It often zooms too fast, which can make it difficult for transcribers to see the text they're transcribing.

We should look into:

• Zoom settings for the current library we use for displaying the page images (OpenSeadragon?); or
• Other, better libraries/gems we could use to display page images