Add option to keep evidence when suspending accounts

Fix #547

When selected, before the account's data is removed, some of it
is denormalized into a separate, symmetrically-encrypted table. In
particular:

- The e-mail
- All IPs used to access the account
- SHA256 fingerprints of all uploaded files
- URIs of accounts followed by or following the account
- URIs of accounts that were invited

app/controllers/settings/deletes_controller.rb

@@ -46,7 +46,7 @@ class Settings::DeletesController < Settings::BaseController
 
   def destroy_account!
     current_account.suspend!
-    Admin::SuspensionWorker.perform_async(current_user.account_id, true)
+    Admin::SuspensionWorker.perform_async(current_user.account_id, reserve_email: false)
     sign_out
   end
 end