Fixes to Require Data Types and Restrict the Selection Dropdown

The fixes here ensure that a new data value cannot be saved without a data type.  And we ensure app/views/data/_form.html.haml  her user's data types in the dropdown.

Summary of changes by file:

`app/controllers/data_controller.rb` - Fix page refresh when errors occur.
`app/models/datum.rb` - Always require a data type before saving.
`app/views/data/_form.html.haml` - Restrict the dropdown to the logged in user's data types.
`app/views/data/index.html.haml` - Display blank rather than crashing if somehow a null is saved into the data type.