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

db/migrate/20200112170923_create_secure_account_summaries.rb

@@ -0,0 +1,11 @@
+class CreateSecureAccountSummaries < ActiveRecord::Migration[5.2]
+  def change
+    create_table :secure_account_summaries do |t|
+      t.bigint :account_id, index: true
+      t.string :encrypted_summary, default: '', null: false
+      t.string :encrypted_summary_iv, default: '', null: false, index: { unique: true }
+
+      t.timestamps
+    end
+  end
+end