Add options with custom URL slug and taxonomy name

In response to [this support ticket](https://wordpress.org/support/topic/change-staff-member-to-something-else) and my own needs, I added a new options page to the dashboard menu with the options to change the URL slug and rename the taxonomy. The appearance is ugly, but I added css classes to the form to make it easily styled. Translations were not included, but the hooks for translation should be in place.