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>
Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
similar to https://github.com/inveniosoftware/invenio/pull/2386/files#r18590170