Add table UI for adding new codes and code descriptions

[elinw]

Currently codes are read in from a csv file and new codes (but not descriptions) can be added while coding a document. This will add a way to add or edit a description and a way to create codes from a table view.

[elinw]

Since we have switched to using datatable we should be able to do this with a combination of the table option and then code for saving. rstudio/DT#480

[elinw]

Did this by adding to the "add data" tab for now. Maybe we will add it to the codes tab or do something else that is more visually appealing as an enhancement later.