Track travel advice searches

Adds analytics tracking to the travel advice country filter.

Part of:
https://www.agileplannerapp.com/boards/105200/cards/5712
It would be good to store the timeout on the instace rather than the global CountryFilter object. So something like:

```javascript
CountryFilter.prototype._trackTimeout = false;

CountryFilter.prototype.track = function(search) {
  clearTimeout(this._trackTimeout);
  this._trackTimeout = root.setTimeout(function(){
    ...
  }, 1000);
```