Rename 'Include nested category levels:' to 'Subcategory depth:'

[trnstlntk]

During user testing, @lozanaross asked users (without giving them instructions) to guess what the subcategory depth input field meant. Many people thought it was a checkbox and most couldn't guess what it was supposed to do (especially people unfamiliar with other tools to work with Wikimedia Commons categories).

We suggest to rename the Include nested category levels: text in the interface to Subcategory depth: which is a bit shorter, and gives more indication that a number needs to be entered in the input field.

[j-sal]

Closed as per #66