Fix various columns to be either `character varying` or `text`,
depending on what kind of text is stored in the column. `text` is used
for columns that contain free-form natural language, like pool and forum
topic titles, while `character varying` is used for short strings that
don't contain free-form text, like URLs and status fields.
Both types are treated the same by Postgres; the only difference is how
we treat them in Rails. In edit forms, `text` fields use multi-line
textboxes, while `character varying` fields use single-line inputs. And
during search, we allow `text` fields to be searched using full-text
search, but not `character varying` fields.
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