WebSubmit: Autocompeltion for authors

* Provides an author autocompletion facility for WebSubmit. (closes #2276)

* The author autocompletion interface can easily be configured for each
  submission.

* Author sources can be defined as plugins that return results in JSON.

* Author submission and modification are fully compatible with existing
  BibConvert templates.

* Uses typeahead.js <http://twitter.github.io/typeahead.js/>
  for autocompletion.

Co-Authored-By: Harris Tzovanakis <charalampos.tzovanakis@cern.ch>
Co-Authored-By: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>