Refresh webfinger (#1323)

* Refresh local info for remote accounts when webfinger returns new values

It only refreshes account info if one of the URLs or the public-key changes,
in which cases it refreshes the full info, re-downloading the feeds from that
user.

Some special handling should probably be done when the public key changes,
but I have been unable to find any use for it in Mastodon yet.

* Re-fetch remote users we aren't subscribed to.

This might induce performance issues, we might want to only do that for users
we explicitly attempted to subscribe but failed to.

* Refactor changes

* Do not refresh existing remote account details more than once a day

* Avoid re-fetching webfinger info in tests unless otherwise specified

db/migrate/20170409170753_add_last_webfingered_at_to_accounts.rb

@@ -0,0 +1,5 @@
+class AddLastWebfingeredAtToAccounts < ActiveRecord::Migration[5.0]
+  def change
+    add_column :accounts, :last_webfingered_at, :datetime
+  end
+end

db/schema.rb

@@ -40,6 +40,7 @@ ActiveRecord::Schema.define(version: 20170406215816) do
     t.datetime "header_updated_at"
     t.string   "avatar_remote_url"
     t.datetime "subscription_expires_at"
+    t.datetime "last_webfingered_at"
     t.boolean  "silenced",                default: false, null: false
     t.boolean  "suspended",               default: false, null: false
     t.boolean  "locked",                  default: false, null: false