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/models/secure_account_summary.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class SecureAccountSummary < ApplicationRecord
+  belongs_to :account
+  attr_encrypted :summary, key: Rails.configuration.x.otp_secret[0...32]
+end