Change hashtags to preserve first-used casing (#11416)

db/migrate/20190726175042_add_case_insensitive_index_to_tags.rb

@@ -0,0 +1,15 @@
+class AddCaseInsensitiveIndexToTags < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    safety_assured { execute 'CREATE UNIQUE INDEX CONCURRENTLY index_tags_on_name_lower ON tags (lower(name))' }
+    remove_index :tags, name: 'index_tags_on_name'
+    remove_index :tags, name: 'hashtag_search_index'
+  end
+
+  def down
+    add_index :tags, :name, unique: true, algorithm: :concurrently
+    safety_assured { execute 'CREATE INDEX CONCURRENTLY hashtag_search_index ON tags (name text_pattern_ops)' }
+    remove_index :tags, name: 'index_tags_on_name_lower'
+  end
+end