adding ms photos to pb elements with @facs

[ebeshero]

@brookestewart will be looking for the best photo (or photos) of each letter page to point to on the <pb/> elements in the ms letters, starting with the 1823 directory. This will involve applying the @facs attribute with the attribute value being the file name of the image. If you want to point to multiple images, separate them with a white space, but try to be selective of the best 1 to 3 images per page.

@ebeshero will then write code for the website rendering of the letters to point to thumbnails of the ms images that people can enlarge.

[ebeshero]

We are now ready to move forward with this!