Make the compose area optionally scrollable.

On desktop, the compose text box grows to accommodate the content.  On
mobile, the text box does not grow to accommodate text context, but does
grow to accommodate images.  It is possible in both cases to overflow
the available area, which makes accessing other UI elements (e.g.
visibility setttings) difficult.

This commit makes the compose area optionally scrollable, which allows
those UI elements to remain available even if they go off-screen.

app/javascript/mastodon/features/compose/index.js

@@ -105,7 +105,7 @@ export default class Compose extends React.PureComponent {
         <SearchContainer />
 
         <div className='drawer__pager'>
-          <div className='drawer__inner' onFocus={this.onFocus}>
+          <div className='drawer__inner scrollable optionally-scrollable' onFocus={this.onFocus}>
             <NavigationContainer onClose={this.onBlur} />
             <ComposeFormContainer />
           </div>