FIX: Prevent column name conflicts in reviewable code (PR #9753)

We were getting errors like this in Reviewables in some cases:

ActiveRecord::StatementInvalid (PG::AmbiguousColumn: ERROR:  column reference "category_id" is ambiguous
LINE 4: ...TRUE) OR (reviewable_by_group_id IN (NULL))) AND (category_i...

The problem that was making everything go :boom: is that plugins can add their own custom filters for Reviewables. If one is doing an INNER JOIN on topics, which has its own category_id column, we would get the above AmbiguousColumn error. The solution here is to just make all references to the reviewable columns in the list_for and viewable_by code prefixed by the table name e.g. reviewables.category_id.

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