remove BASE_URL from getBlockTypeAssetsURL

avoid having http:// in these includes when a site runs with https://
Allow overriding package's blocks assets (view.js & view.css)
Allow overriding package's blocks assets (js/*.js & css/*.css)
Change ConcreteUrlsHelper::getBlockTypeAssetsURL to return relative urls for block's assets
Merge pull request #1391 from Remo/patch-56

remove BASE_URL from getBlockTypeAssetsURL
Merge pull request #1419 from ojalehto/fix/package-block-assets/allow-overrides

Allow overriding package's block's assets