Implement dashboard filters

This commit includes some refactoring.

* Store references to the filters as well as rendered translation list elements
* Removed unused parameter in getLinkImage
* Add listen(), execute render() earlier
* Populate language filters with values present in translations
* Helper function for filter creation

The filters are additive (all must match).

Possible improvements in the future:
* Use translated language name
* Simplify the format returned by the API
* Use ULS for the language filters
* Display a message when filters hide all items
* Show a spinner while list items are being loaded via API

Change-Id: I7da54754c511ab2a742594b87d52facba654c577
Merge "Implement dashboard filters"