Dataset - change filter type

[yochannah]

For dataset, it might be easier to show a dropdown containing all options (there usually aren't that many, e.g. 10-20 datasets), or a set of checkboxes that allows multiple datasets to be chosen at once.

For the checkbox option, we'd need to sort alphabetically and show only the top 5, with the option to "view more". Checkbox implementation might be related to #41

[AdrianBZG]

@yochannah I will check if I can query using offset for the dataset names.

[AdrianBZG]

@yochannah Done!: http://im-browser-prototype.herokuapp.com/