ip bans: add hit counter, deleted flag, new ban type.

* Make IP bans soft deletable.
* Add a hit counter to track how many times an IP ban has blocked someone.
* Add a last hit timestamp to track when the IP ban last blocked someone.
* Add a new type of IP ban, the signup ban. Signup bans restrict new
  signups from editing anything until they've verified their email
  address.

db/migrate/20200406054838_add_type_to_ip_bans.rb

@@ -0,0 +1,11 @@
+class AddTypeToIpBans < ActiveRecord::Migration[6.0]
+  def change
+    add_column :ip_bans, :is_deleted, :boolean, default: false, null: false
+    add_column :ip_bans, :category, :integer, default: 0, null: false
+    add_column :ip_bans, :hit_count, :integer, default: 0, null: false
+    add_column :ip_bans, :last_hit_at, :datetime
+
+    add_index :ip_bans, :is_deleted
+    add_index :ip_bans, :category
+  end
+end