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

spec/fabricators/secure_account_summary_fabricator.rb

@@ -0,0 +1,4 @@
+Fabricator(:secure_account_summary) do
+  account
+  summary "{}"
+end

spec/models/secure_account_summary_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe SecureAccountSummary, type: :model do
+end