Admin Tag Autocompletion

Adds Tag name autocomplete functionality to New and Edit Article forms.

* :star: Added jquery-ui components
* :star: New data transformer `NewTagsTransformer`
 * transforms a comma delimited string of tag names into `Tag` objects
 * reverse-transforms `Tag` objects into a comma delimited string of tag names
* :memo: Template `AdminBundle::base.html.twig` extends `MainBundle::base_notrack.html.twig`
 * use as base template for all AdminBundle templates
* :star: New resource `js/tag_autocomplete.js`
 * Handles autocomplete functionality
 * Adds new tags to `new_tags` form field when user adds a new tag
 * Checks/unchecks existing tags on `tags` form field when user adds/removes an existing tag