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/post-thread/PostThread.tsx

@@ -6,7 +6,7 @@ import {
   PostThreadViewPostModel,
 } from '../../../state/models/post-thread-view'
 import {PostThreadItem} from './PostThreadItem'
-import {ErrorMessage} from '../util/ErrorMessage'
+import {ErrorMessage} from '../util/error/ErrorMessage'
 
 export const PostThread = observer(function PostThread({
   uri,
@@ -57,7 +57,6 @@ export const PostThread = observer(function PostThread({
     return (
       <View>
         <ErrorMessage
-          dark
           message={view.error}
           style={{margin: 6}}
           onPressTryAgain={onRefresh}