saved searches: fix normalization of labels.

Fix saved searches to remove additional invalid characters from labels:

* Remove repeated spaces or underscores.
* Remove leading and trailing spaces or underscores.
* Normalize Unicode characters to NFC form.

Also add a fix script to renormalize labels in old saved searches. A few
problems with existing searches:

* Some saved searches somehow had labels containing NULL elements.
* Some had leading or trailing underscores.
* Some had repeated underscores.
* Some had non-English characters in uppercase.

script/fixes/070_renormalize_labels_in_saved_searches.rb

@@ -0,0 +1,15 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+SavedSearch.transaction do
+  SavedSearch.find_each do |ss|
+    old_labels = ss.labels
+    ss.labels = old_labels
+
+    if ss.changed?
+      p [ss.id, ss.changes]
+      ss.save!
+    end
+  end
+end