Add "all" to filter dropdown and style it up

[Methodician]

Filter dropdown should include option like "all" or "remove" or whatever is good UX. Should also be a little prettier. Who likes an ugly dropdown?

I'm thinking: try just putting a remove button or a radio or checkbox next to it, which would be deactivated or disappear if no filter is set. Just a thought...

[m-chu]

Added option to return to All Posts after initial category selection.

[m-chu]

Updated selector design with Angular Material.