taxonomy filter with depth completely broken

Problem/Motivation

The 'Has taxonomy terms (with depth)' filter does not modify the query at all.

To reproduce:

1. Add 3 terms to the 'tags' vocab, and nest them under each other
2. Create a view of article content
3. Add the 'Has taxonomy terms (with depth)', and set depth to 3
4. Update the view preview and note the SQL does not change at all (eg, the filter is not applied)

Proposed resolution

Fix the filter and add tests.

Remaining tasks

User interface changes

API changes