Store URIs of follows, follow requests and blocks for ActivityPub (#7160)

Same URI passed between follow request and follow, since they are
the same thing in ActivityPub. Local URIs are generated during
creation using UUIDs and are passed to serializers.

db/schema.rb

@@ -10,10 +10,9 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2018_04_10_204633) do
+ActiveRecord::Schema.define(version: 2018_04_16_210259) do
 
   # These are extensions that must be enabled in order to support this database
-  enable_extension "pg_stat_statements"
   enable_extension "plpgsql"
 
   create_table "account_domain_blocks", force: :cascade do |t|
@@ -112,6 +111,7 @@ ActiveRecord::Schema.define(version: 2018_04_10_204633) do
     t.datetime "updated_at", null: false
     t.bigint "account_id", null: false
     t.bigint "target_account_id", null: false
+    t.string "uri"
     t.index ["account_id", "target_account_id"], name: "index_blocks_on_account_id_and_target_account_id", unique: true
   end
 
@@ -176,6 +176,7 @@ ActiveRecord::Schema.define(version: 2018_04_10_204633) do
     t.bigint "account_id", null: false
     t.bigint "target_account_id", null: false
     t.boolean "show_reblogs", default: true, null: false
+    t.string "uri"
     t.index ["account_id", "target_account_id"], name: "index_follow_requests_on_account_id_and_target_account_id", unique: true
   end
 
@@ -185,6 +186,7 @@ ActiveRecord::Schema.define(version: 2018_04_10_204633) do
     t.bigint "account_id", null: false
     t.bigint "target_account_id", null: false
     t.boolean "show_reblogs", default: true, null: false
+    t.string "uri"
     t.index ["account_id", "target_account_id"], name: "index_follows_on_account_id_and_target_account_id", unique: true
   end