New download tab

- Add new markup and styles for download-tab
     - Add poster image with overlay-image
     - Display media data like duration and published date
     - Add footer with download button and file-format select bar
     - Add overlay images for download-tab's episode image
- Apply correct font-family for \<h1\> tag in 'tabs' 
- Specify flexbox-hack to avoid unintended behavior
- Add general styles for \<select\> elements
- Change gray color variables (names and values)
- Import and apply new font-families ‘Roboto’ and ‘Droid Sans Mono’
- Delete unused font-weights
- Rename class .button-subscribe to .button-submit
- Rename class .image-container to .poster-image