Add a design system (#34)

* Add theming system

* Add standard Button control and update RadioButtons

* Unify radiobutton with design system

* Update debug screen to have multiple views

* Add ToggleButton

* Update error controls to use design system

* Add typography to <Text> element

* Move DropdownButton into the design system

* Clean out old code

* Move Text into design system

* Add 'inverted' color palette

* Move LoadingPlaceholder into the design system

src/view/com/util/ViewHeader.tsx

@@ -8,7 +8,7 @@ import {
 } from 'react-native'
 import {FontAwesomeIcon} from '@fortawesome/react-native-fontawesome'
 import {UserAvatar} from './UserAvatar'
-import {Text} from './Text'
+import {Text} from './text/Text'
 import {s, colors} from '../../lib/styles'
 import {MagnifyingGlassIcon} from '../../lib/icons'
 import {useStores} from '../../../state'