🧹 Add task to remove placeholder file sets

This commit will add a rake task to remove placeholder file sets from
the Dickinson tenant.  To test, set an environment variable with
whatever cname you want to use and run the task.  For example:

```sh
REMOVE_PLACEHOLDER_FILE_SETS_FROM="dev.hyku.test" rake hyku:remove_placeholder_file_sets
```

Ref:
  - #912

lib/tasks/remove_dickinson_placeholder_thumbnails.rake

@@ -0,0 +1,28 @@
+namespace :hyku do
+  desc "Remove Hyku Commons placeholder file sets from tenant"
+  task remove_placeholder_file_sets: :environment do
+    # The original ask was for the Dickinson tenant, but passing in an ENV var would be useful for testing
+    # example:
+    # ```sh
+    # REMOVE_PLACEHOLDER_FILE_SETS_FROM="dev.hyku.test" rake hyku:remove_placeholder_file_sets
+    # ```
+    account_cname = ENV['REMOVE_PLACEHOLDER_FILE_SETS_FROM'] || "dickinson.hykucommons.org"
+
+    Account.find_each do |account|
+      next unless account.cname == account_cname
+
+      begin
+        switch!(account.cname)
+        puts "********************** switched to #{account.cname} **********************"
+
+        file_set_titles = ["HykuCommonsPlaceholder.pdf", "hykuplaceholderarchives.pdf"]
+        query = file_set_titles.map { |title| "title_tesim:\"#{title}\"" }.join(" OR ")
+        file_sets = FileSet.where(query)
+        file_sets.each { |file_set| file_set.destroy(eradicate: true) }
+      rescue StandardError => e
+        puts "********************** error: #{e} **********************"
+        next
+      end
+    end
+  end
+end