react [nfc]: Use PureComponent where we say we do

In docs/architecture/react.md we say we derive all our class
components from React.PureComponent, with one exception, namely
MessageList.

From a grep, there are a handful of other components where we use
simply React.Component instead.  I'm not sure the difference has
much practical effect on those components, but adjust them to follow
our pattern for consistency.

None of these components need special behavior in this respect:
they don't implement shouldComponentUpdate, and just like for other
components in our app the values we pass for their props are never
mutated, only replaced with new values.

Author: gnprice

src/RootErrorBoundary.js

@@ -34,7 +34,7 @@ type State = $ReadOnly<{|
  *
  * [1] https://reactjs.org/docs/error-boundaries.html#how-about-event-handlers
  */
-export default class ErrorBoundary extends React.Component<Props, State> {
+export default class ErrorBoundary extends React.PureComponent<Props, State> {
   static getDerivedStateFromError(error: Error): State {
     return { error };
   }

src/sharing/ShareToPm.js

@@ -37,7 +37,7 @@ type State = $ReadOnly<{|
   choosingRecipients: boolean,
 |}>;
 
-export default class ShareToPm extends React.Component<Props, State> {
+export default class ShareToPm extends React.PureComponent<Props, State> {
   state: State = {
     selectedRecipients: [],
     choosingRecipients: false,

src/sharing/ShareToStream.js

@@ -47,7 +47,7 @@ type State = $ReadOnly<{|
   isTopicFocused: boolean,
 |}>;
 
-class ShareToStreamInner extends React.Component<Props, State> {
+class ShareToStreamInner extends React.PureComponent<Props, State> {
   state = {
     streamName: '',
     streamId: null,

src/sharing/ShareWrapper.js

@@ -98,7 +98,7 @@ type State = $ReadOnly<{|
  * Wraps Around different sharing screens,
  * for minimal duplication of code.
  */
-class ShareWrapperInner extends React.Component<Props, State> {
+class ShareWrapperInner extends React.PureComponent<Props, State> {
   static contextType = TranslationContext;
   context: GetText;