MySQL search

Add a single search_index table for full-text search of cards and
comments.

For the search there is a full-text index on the title and content
columns. The board_ids is also included in the table and accessible
board ids are pre-loaded and included in the search query. This allows
us to filter out inaccessible records before joining with other tables.

Right now the search is just using boolean search. This would give us
a bunch of syntax options
(see https://dev.mysql.com/doc/refman/8.4/en/fulltext-boolean.html)
except the search query filters those out.

I've removed the searchable_by method for now - everything is built
on the assumption that there's a single search index table and all data
must fit into it.

Queries are written in SQL, we don't have a SearchIndex ActiveRecord
model. That's because we'll likely want to shard the table and it will
be simpler to just keep with the raw SQL for that.

There's no stemming, highlighting or snippet extraction yet - we are
dumping the full description in the search results.

Data can be reindexed with the search:reindex rake task.

lib/tasks/search.rake

@@ -0,0 +1,19 @@
+namespace :search do
+  desc "Reindex all cards and comments in the search index"
+  task reindex: :environment do
+    puts "Clearing search index..."
+    ActiveRecord::Base.connection.execute("DELETE FROM search_index")
+
+    puts "Reindexing cards..."
+    Card.find_each do |card|
+      card.reindex
+    end
+
+    puts "Reindexing comments..."
+    Comment.find_each do |comment|
+      comment.reindex
+    end
+
+    puts "Done! Reindexed #{Card.count} cards and #{Comment.count} comments."
+  end
+end