Hide watched `movie-item`s

https://github.com/popcorn-official/popcorn-app/issues/844

This is only half of the implementation, since the showing/hiding of watched videos can only be controlled through code. It worked fine for me so far.

Idea for the second half: We could make the watched toggle 3-state instead of 2:
1) Show all videos, including watched
2) Dim (e.g. `opacity 0.5`) watched videos so new ones stand out.
3) Hide watched videos completely (`display: none`, or somehow even exclude them from the response from the server) for maximum focus.

I would imagine a toggle like this to be similar to the heart icon we have in the top-right corner: A small icon-only-button, which toggles between the 3 states when clicked, and in appearance, represents the current selected state.

### Aspects left to consider:

When toggling between watched states, the viewport will jump up and down from the use's perspective based on movies being shown/hidden above the viewport.

```Gherkin
Given I see the movie list
Given I scrolled down from the top
Given I take note of the first movie I see in the viewport
When I toggle watched state
Then the first movie I see in the viewport should be the same
```
(Use-case written in the Gherkin language)

We'll need additional logic to accomplish this, or something similar.