Add Next/Previous buttons to Observation vetting screen

In an effort to make the vetting process as efficient as possible, it'd be handy to have Next and Previous buttons on the Observation vetting screen.

• Should fail gracefully (return to list screen) if at the beginning/end of list.
• Should be aware of filters applied when the list of observations was created and stay within those filters.
• Should gracefully handle items being added/removed from the list while viewing (IE - a new observation is added, an observation is updated that removes it from current filtered list)