Add ability to sort contents by date published, last seen, and alphabetically

Addresses https://github.com/nbproject/nbproject/issues/54

<del>Currently displays "Date Added" in table with the ability to sort by clicking the header.</del>

<del>If we want to add the ability to sort by date viewed, I'm guessing we don't want that information in the table (too much clutter). Would it make sense to have a line above the table that says "Sort by: last seen, date added, alphabetical" where you can just click the words to sort? Should it default to sorting by last seen?</del>

<del>Defaults to sorting by most recently viewed first.</del>

<del>Currently page views don't get tracked for HTML pages, so HTML pages will always remain at the bottom when sorting by "last seen". </del>

4/25: "Last seen" is implemented but refers to "last seen by any user," not just the viewer. Currently the two options for sorting are by "date added" and "name." Defaults to sorting by name.