Add base flowplayer styles.

@incuna/frontend please.
YouTube sets the correct dimensions of the video to the `<video>` element, and has a fixed container `<div>` element. Here you're styling the `<video>` element, and that will produce stretching in some browsers (I think some will ignore CSS and only care about HTML attributes.)

You could have a class to determine the dimension along which you want to restrict. For a page of content, that's `width`; for a horizontal strip, that's `height`. You can optionally restrict both, even though it would potentially squash or stretch the video.
```html
<!-- Class can be set by the directive -->
<div flowplayer class="restrict-width">
    <video>
        <source src="video.mp4"></source>
    </video>
</div>
```
```sass
[flowplayer]
    &.restrict-height
        max-height: 100%
    &.restrict-width
        max-width: 100%
    &.restrict-both
        max-height: 100%
        max-width: 100%
```