tweak Doctor modal footer UI, replace deprecated UI components (#3331)

Summary:
This PR fixes the unaligned checkbox in the Doctor modal (I have based the solution of the Settings modal code) and also replaces deprecated components `FlexRow` and `FlexColumn` in Settings modal.

## Changelog

* fix checkbox placement in the Doctor modal footer

Pull Request resolved: https://github.com/facebook/flipper/pull/3331

Test Plan:
The change has been testes by running the desktop Flipper app locally from source.

## Preview (before & after)

<img width="596" alt="Screenshot 2022-01-23 at 16 01 48" src="https://user-images.githubusercontent.com/719641/150685091-ce494847-eea6-4263-900a-2b5541d3d897.png">
<img width="596" alt="Screenshot 2022-01-23 at 16 01 44" src="https://user-images.githubusercontent.com/719641/150685096-29723d14-6cfc-42aa-9564-431a61b1061d.png">

Reviewed By: aigoncharov

Differential Revision: D33738683

Pulled By: lblasa

fbshipit-source-id: 900657d6cb095d6d32dd412205e714a46e64a564

desktop/flipper-ui-core/src/sandy-chrome/WelcomeScreen.tsx

@@ -8,7 +8,7 @@
  */
 
 import React, {cloneElement} from 'react';
-import {styled, FlexRow, FlexColumn} from '../ui';
+import {styled} from '../ui';
 import {Modal, Button, Image, Checkbox, Space, Typography, Tooltip} from 'antd';
 import {
   RocketOutlined,
@@ -29,7 +29,7 @@ import {getFlipperLib} from 'flipper-plugin';
 import {ReleaseChannel} from 'flipper-common';
 import {showChangelog} from '../chrome/ChangelogSheet';
 
-const RowContainer = styled(FlexRow)({
+const RowContainer = styled(Layout.Horizontal)({
   alignItems: 'flex-start',
   padding: `${theme.space.small}px`,
   cursor: 'pointer',
@@ -53,19 +53,19 @@ function Row(props: {
           {cloneElement(props.icon, {
             style: {fontSize: 36, color: theme.primaryColor},
           })}
-          <FlexColumn>
+          <Layout.Container>
             <Title level={3} style={{color: theme.primaryColor}}>
               {props.title}
             </Title>
             <Text type="secondary">{props.subtitle}</Text>
-          </FlexColumn>
+          </Layout.Container>
         </Space>
       </RowContainer>
     </Tracked>
   );
 }
 
-const FooterContainer = styled(FlexRow)({
+const FooterContainer = styled(Layout.Horizontal)({
   justifyContent: 'space-between',
   alignItems: 'center',
 });