Enable Multi-Image content block

Display multi-image block using bxSlider.
Extend help text for promotion models
Add more links to promotions dashboard templates
Check for txSlider existance before trying to use it

The dashboard templates don't load it so the old code raises an error.
Ignore verify_exists for local URL validation

There's no security issue with checking local URLs.
Serve (full) jQuery UI form CDN

The customised version didn't have sorting which is required for the
promotions dashboard.  We use the full file from a CDN.
Merge pull request #953 from tangentlabs/feature/multiimage

Enable Multi-Image content block