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>
1: D100 Docstring missing
22:1: F401 'get_dictionary_from_string' imported but unused
32:1: E302 expected 2 blank lines, found 1
32: D103 Docstring missing
41:1: E302 expected 2 blank lines, found 1
63:1: E302 expected 2 blank lines, found 1
63: D401 First line should be imperative: 'Convert', not 'Converts'
63: D202 No blank lines allowed *after* function docstring, found 1
63: D400 First line should end with '.', not 'N'
63: D205 Blank line missing between one-line summary and description
75:5: F841 local variable 'field_key_to_separator' is assigned to but never used
96:48: E231 missing whitespace after ','
101:43: E231 missing whitespace after ','
101:50: E231 missing whitespace after ','