ZulipButton [nfc]: Limit the set of style properties allowed.

The use of specific style properties for callers to micromanage the
layout of the button is pretty chaotic and fragile, introducing
implicit dependencies on details of the ZulipButton implementation.

It'd be good to assimilate those usages into a more coherent set of
options provided by the ZulipButton interface explicitly.  This
would also help with making the buttons' appearance more consistent
across the app.

Until then, by listing here the properties we do use, we can at least
make it possible when working on the ZulipButton implementation to
know the scope of different ways that callers can mess with the styles.

src/common/ZulipButton.js

@@ -1,10 +1,11 @@
 /* @flow strict-local */
 import React, { PureComponent } from 'react';
 import { Text, View, ActivityIndicator } from 'react-native';
-import type { TextStyleProp, ViewStyleProp } from 'react-native/Libraries/StyleSheet/StyleSheet';
+import type { TextStyle, ViewStyle } from 'react-native/Libraries/StyleSheet/StyleSheet';
 import TranslatedText from './TranslatedText';
 
 import type { LocalizableText } from '../types';
+import type { SubsetProperties } from '../generics';
 import type { SpecificIconType } from './Icons';
 import { BRAND_COLOR, createStyleSheet } from '../styles';
 import Touchable from './Touchable';
@@ -63,8 +64,38 @@ const styles = createStyleSheet({
 });
 
 type Props = $ReadOnly<{|
-  style?: ViewStyleProp,
-  textStyle?: TextStyleProp,
+  /* eslint-disable flowtype/generic-spacing */
+  style?: SubsetProperties<
+    ViewStyle,
+    {|
+      // The use of specific style properties for callers to micromanage the
+      // layout of the button is pretty chaotic and fragile, introducing
+      // implicit dependencies on details of the ZulipButton implementation.
+      // TODO: Assimilate those usages into a more coherent set of options
+      //   provided by the ZulipButton interface explicitly.
+      //
+      // Until then, by listing here the properties we do use, we at least
+      // make it possible when working on the ZulipButton implementation to
+      // know the scope of different ways that callers can mess with the
+      // styles.  If you need one not listed here and it's in the same
+      // spirit as others that are, feel free to add it.
+      margin?: mixed,
+      marginTop?: mixed,
+      marginBottom?: mixed,
+      marginHorizontal?: mixed,
+      // (... e.g., go ahead and add more variations on margin.)
+      padding?: mixed,
+      paddingLeft?: mixed,
+      paddingRight?: mixed,
+      height?: mixed,
+      width?: mixed,
+      borderRadius?: mixed,
+      flex?: mixed,
+      alignSelf?: mixed,
+      backgroundColor?: mixed,
+    |},
+  >,
+  textStyle?: SubsetProperties<TextStyle, {| color?: mixed |}>,
   progress: boolean,
   disabled: boolean,
   Icon?: SpecificIconType,