In SQL, conditions like:
```
tag_id IN (x) AND taggable_type='Debate' OR taggable_type='Proposal'
```
Don't work as intended; we need to write:
```
tag_id IN (x) AND (taggable_type='Debate' OR taggable_type='Proposal')
```
Due to this bug, we were returning taggings for proposals without
intending to do so.
Since the code was very hard to read, we're also simplifying it.
2e863fdc51