Centerer [nfc]: Add TODO to delete

And write in a code comment something we noticed in review of zulip#4868
("Centerer: Remove, as unnecessary") about how we might like
`Screen` to work.

With this, I plan to close zulip#4868. It's stale, and when looking
through my open PRs, it takes more attention than it deserves.

src/common/Centerer.js

@@ -33,6 +33,9 @@ type Props = $ReadOnly<{|
  * @prop children - Components to be centered.
  * @prop [padding] - Specifies if the components should be padded.
  */
+// TODO: Remove this component. Callers should be more transparent about
+//   what they want, especially if that includes setting things unrelated to
+//   centering, like `maxWidth: 480`.
 export default class Centerer extends PureComponent<Props> {
   static defaultProps: {| padding: boolean |} = {
     padding: false,

src/common/Screen.js

@@ -90,6 +90,8 @@ export default function Screen(props: Props): Node {
         centerContent: {
           flexGrow: 1,
           justifyContent: 'center',
+          // TODO: decide whether to add `alignItems: 'center'`:
+          //   https://github.com/zulip/zulip-mobile/pull/4868#discussion_r665486501
         },
         padding: {
           padding: 16,