[#1792] Styling filters
[#1792] Allow clearing filters

Unfortunately, select2 require not only setting `allowClear`, but also having a
`placeholder`. As I don't want to have a `placeholder`, I've set it to `' '` as
a workaround.
[#1792] Set the filters' width using CSS
[#1792] Keep filters persistent when changing across views

The idea is simple. I changed the links for the other resource views of the
same resource to keep the `filters` query string, if it exist. I do this even
for resource views that aren't filterable, so even if you go from resource view
Foo, which is filterable, and set some filters, then move to resource view Bar
which isn't filterable, then to resource view Baz which is filterable, the
filters you've set in Foo are still set when you visit Baz.
[#1792] Add polyfill for Object.keys() and remove usage of Array.filter

We need this to make sure the code works on older browsers.
[#1792] Minor refactorings
[#1792] Don't set filters on links when there're no filters
[#1792] Fix bug where filters weren't appearing
[#1792] Fix bug where we were unable to remove filters

Instead of removing the filters, we were setting them to `undefined`.