[Feature] Show selected categories and option to clear all selected items

[kristenwomack]

Is your feature request related to a problem? Please describe.
We have a lot of filter groups and some of them are fairly long. Once you select one or more items, you can "lose" what you've selected.

Describe the solution you'd like
It would be ideal to have the visual tags showing what is currently selected and the option to clear all selected filters. See embedded image here.

cc: @Austinauth

[Austinauth]

This goes back to the faceted filtering we never got around to implementing.

Ideally the user wouldn't be able to select a combination of filters that do not yield a result. For example: Python & Javascript.

Ideally we'd have more time to make this look a little nicer, but since we've added so many new filters, it also might be nice to add a clear all button next to the "Filter By" text (only when the user has selected a filter).