Convert related artefact UI to set of dropdowns

This is a stepping stone to making them reorderable.
Once they are reorderable, we can reinstate the augmentation (select2),
as it requires making significant changes.
Use jquery from gem rather than including source

Want to use the jquery-ui-rails gem to cleanly use parts of jquery-ui, but that
depends on jquery-rails.

This should be cleaner anyhow.
Related artefacts can now be reordered
Avoid repeatedly querying the list of artefacts

Big performance improvement.
'Add another related item' clones from a template

This will make the change to allow deleting of every related item easier.
Allow removal of all related items
Merge pull request #53 from alphagov/allow_ordering_of_related_items

Allow ordering of related items