Allow import/export of mutes list (#1541)

* Allow export of mutes list

* Allow importing of mutes list

* Refactor to use Settings::Exports::BaseController and DRY up exports code

spec/controllers/settings/exports/muted_accounts_controller_spec.rb

@@ -0,0 +1,17 @@
+require 'rails_helper'
+
+describe Settings::Exports::MutedAccountsController do
+  before do
+    sign_in Fabricate(:user), scope: :user
+  end
+
+  describe 'GET #index' do
+    it 'returns a csv of the muting accounts' do
+      get :index, format: :csv
+
+      expect(response).to have_http_status(:success)
+      expect(response.content_type).to eq 'text/csv'
+      expect(response.headers['Content-Disposition']).to eq 'attachment; filename="muted_accounts.csv"'
+    end
+  end
+end