Set up a default cookie

Set up a default cookie key name of "mongo-express" to prevent it from
overwriting cookies for other express apps using the default cookie
name.
Moved the cookie key into the config

Moved the cookie key name into the config to make it more obvious that
it's something that can be customized.
Enable search by ObjectID

Enable search by ObjectID. TODO: validation to prevent errors with
invalid objectID
Updated ObjectID search

Updated ObjectID search to allow for multiple formats and turned on
basic validation to prevent crashes. Error message is flashed to the
user.
debug