db: fix types of various string columns.

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.

app/views/forum_topics/_form.html.erb

@@ -1,6 +1,6 @@
 <div id="form-content">
   <%= edit_form_for(forum_topic) do |f| %>
-    <%= f.input :title %>
+    <%= f.input :title, as: :string %>
 
     <div class="input">
       <label for="forum_topic_category_id">Category</label>