Account switcher (#85)

* Update the account-create and signin views to use the design system.

Also:
- Add borderDark to the theme
- Start to an account selector in the signin flow

* Dark mode fixes in signin ui

* Track multiple active accounts and provide account-switching UI

* Add test tooling for an in-memory pds

* Add complete integration tests for login and the account switcher

__mocks__/@gorhom/bottom-sheet.tsx

@@ -0,0 +1,57 @@
+import React from 'react'
+import {View, ScrollView, Modal, FlatList, TextInput} from 'react-native'
+
+const BottomSheetModalContext = React.createContext(null)
+
+const BottomSheetModalProvider = (props: any) => {
+  return <BottomSheetModalContext.Provider {...props} value={{}} />
+}
+class BottomSheet extends React.Component {
+  snapToIndex() {}
+  snapToPosition() {}
+  expand() {}
+  collapse() {}
+  close() {
+    this.props.onClose?.()
+  }
+  forceClose() {}
+
+  render() {
+    return <View>{this.props.children}</View>
+  }
+}
+const BottomSheetModal = (props: any) => <Modal {...props} />
+
+const BottomSheetBackdrop = (props: any) => <View {...props} />
+const BottomSheetHandle = (props: any) => <View {...props} />
+const BottomSheetFooter = (props: any) => <View {...props} />
+const BottomSheetScrollView = (props: any) => <ScrollView {...props} />
+const BottomSheetFlatList = (props: any) => <FlatList {...props} />
+const BottomSheetTextInput = (props: any) => <TextInput {...props} />
+
+const useBottomSheet = jest.fn()
+const useBottomSheetModal = jest.fn()
+const useBottomSheetSpringConfigs = jest.fn()
+const useBottomSheetTimingConfigs = jest.fn()
+const useBottomSheetInternal = jest.fn()
+const useBottomSheetDynamicSnapPoints = jest.fn()
+
+export {useBottomSheet}
+export {useBottomSheetModal}
+export {useBottomSheetSpringConfigs}
+export {useBottomSheetTimingConfigs}
+export {useBottomSheetInternal}
+export {useBottomSheetDynamicSnapPoints}
+
+export {
+  BottomSheetModalProvider,
+  BottomSheetBackdrop,
+  BottomSheetHandle,
+  BottomSheetModal,
+  BottomSheetFooter,
+  BottomSheetScrollView,
+  BottomSheetFlatList,
+  BottomSheetTextInput,
+}
+
+export default BottomSheet