Updated correspondence metadata, index

I have updated the metadata structure for the correspondence to be consistent with the most recent Archive styling, including options to view XML and cite the page.  I have also added status bars to the index pages for each era of the correspondence, so it is apparent how many letters we have published out of the total number of known letters in a given era, and I have updated the descriptions in the correspondence sections to reflect our most recent estimates of the total number of letters from each era and our recent NHPRC-funded work on the post-Reconstruction letters.

~ Nikki