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

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_12_12_003415) do
+ActiveRecord::Schema.define(version: 2020_01_12_170923) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -196,15 +196,26 @@ ActiveRecord::Schema.define(version: 2019_12_12_003415) do
     t.index ["target_type", "target_id"], name: "index_admin_action_logs_on_target_type_and_target_id"
   end
 
+  create_table "announcements", force: :cascade do |t|
+    t.text "text", default: "", null: false
+    t.boolean "published", default: false, null: false
+    t.boolean "all_day", default: false, null: false
+    t.datetime "scheduled_at"
+    t.datetime "starts_at"
+    t.datetime "ends_at"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+  end
+
   create_table "backups", force: :cascade do |t|
     t.bigint "user_id"
     t.string "dump_file_name"
     t.string "dump_content_type"
-    t.bigint "dump_file_size"
     t.datetime "dump_updated_at"
     t.boolean "processed", default: false, null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.bigint "dump_file_size"
   end
 
   create_table "blocks", force: :cascade do |t|
@@ -614,6 +625,16 @@ ActiveRecord::Schema.define(version: 2019_12_12_003415) do
     t.index ["scheduled_at"], name: "index_scheduled_statuses_on_scheduled_at"
   end
 
+  create_table "secure_account_summaries", force: :cascade do |t|
+    t.bigint "account_id"
+    t.string "encrypted_summary", default: "", null: false
+    t.string "encrypted_summary_iv", default: "", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["account_id"], name: "index_secure_account_summaries_on_account_id"
+    t.index ["encrypted_summary_iv"], name: "index_secure_account_summaries_on_encrypted_summary_iv", unique: true
+  end
+
   create_table "session_activations", force: :cascade do |t|
     t.string "session_id", null: false
     t.datetime "created_at", null: false