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>
26:1: F401 'MagicMock' imported but unused
28:1: E302 expected 2 blank lines, found 1
28: D101 Docstring missing
29: D102 Docstring missing
40: D102 Docstring missing
43:256: E231 missing whitespace after ','
43:477: E231 missing whitespace after ','
43:767: E231 missing whitespace after ','
43:973: E231 missing whitespace after ','
74: D102 Docstring missing