attempted to replace jQuery UI Autocomplete with Typeahead

**⚠ work in progress, not yet ready to merge**

this is an aborted attempt to get rid of jQuery UI

the main issue is that Typeahead doesn't seem to support the distinction between label/caption and value, so selecting a suggested entry always results in the corresponding value (a technical descriptor which shouldn't be exposed in the UI) being written to the respective input field

additional shortcoming compared to the current jQuery UI-based implementation:

* styling is pretty crude
* auto-focus suggestion if there is only a single result (https://github.com/innoq/iqvoc/blob/8be8328cc7109b5516b84cc715ef801f9f8b0704/app/assets/javascripts/iqvoc/entityselect.js.erb#L87)
* prevent form submissions when Enter is pressed (c9077fcffe2de632579af3f77700d2b309da82e9)
Now that `displayKey` works, do we still need these custom templates?