Remove unused TimestampMixin.published_at column

Perhaps at some point in the future it will make sense to add this back.
Update updated time on update

I.e., added onupdate=datetime.now to TimestampMixin.updated_at so that it will
be automatically updated whenever a record is edited.
Show created and updated times on entry pages
Use request.abort(404) instead of import HTTPNotFound
Add slug column to entry table

Entry resources can now be accessed via `id` or `slug`.
Simplify calls to Entry.GET from other resource methods
Reenable delete_entry permission on Entry.GET
Add a way to configure the home page

The home page can be a specific entry or the list of all entries.