Disable rather than hide form fields that can’t be modified

Fixes #1968 

Not clear to the user why the form field is sometimes present and
sometimes not present.

The Category tag field may only be modified if authorities have not yet
been assigned to the Category. This commit explains this and disables
the field if this is true.

![screen shot 2014-11-17 at 12 02 18](https://cloud.githubusercontent.com/assets/282788/5069504/c0d04a8c-6e52-11e4-8f18-78d4064b0498.png)