Added empty results style to typeahead.less #419
Added loader indicator + empty result message in tt-typahead directive (more awesomness in body :dog2:)
All modification are related to #419
+ Now we ask to user if he wants to create a new entity thanks to tt-empty-results-select callback (see below)
+ Added `tt-disable-empty-selection` param to avoid 'create this entity' message in empty result
+ Added `tt-empty-results-selected` callback param to decide what to do on empty result message selected
+ Added `tt-prepend-search-icon` param to, well, prepend the loading/search indicator to the left of the
  search input instead of right.
Added goToContribute method to SearchFormCtrl
This is related to #419: this method is used to handle empty results returned by
typeahead. So when no results could be found we redirect user (if he choses to)
the contribute page so he can create a new entity.
Removed old search indicator from header.dj.html + added needed empty-result-selected callback #419
Implemented new tt-typeaead attributes in topic.contribute.type-relationship.dj.html
Also removed 'Heads up the entity you `etc.`' section from this template. Now we ask
in typeahead if user wants to create a new entity.
Merge branch 'feature/typeahead-loading-indicator' into develop