Bird UI (#34)

* Upload BirdUI

* Update en.yml - BirdUI

Added English localization for BirdUI

* Update themes.yml

* Added SCSS files for BirdUI

* Update themes.yml typo

* Update mastodon-bird-ui-contrast.scss

* Update and rename mastodon-bird-ui-dark.scss to mastodon-bird-ui.scss

* Update mastodon-bird-ui-light.scss

* Delete app/javascript/styles/mastodon-bird-ui-contrast.scss

* Delete app/javascript/styles/mastodon-bird-ui.scss

* Delete app/javascript/styles/mastodon-bird-ui-light.scss

* Re-upload BirdUI due to Ruby failures