Fix two big DOM leaks which were locking Chrome solid.

pillifyLinks leaked Pill components, which if they contained a BaseAvatar
would leak a whole DOM tree retained by the BaseAvatar's onClientSync
event listener.  This tracks the Pill containers so they can be unmounted
via unmountPills.

BasicMessageComposer set an event listener on selectionchange in onFocus
which leaked if onBlur wasn't called.  This removes it in unmount.

We've also seen Velociraptor retaining full DOM trees from RRs, which
this doesn't address as the leak is probably within Velocity, and the plan
is to replace it with CSS animations.

Should fix https://github.com/vector-im/riot-web/issues/12417

src/components/views/rooms/BasicMessageComposer.js

@@ -490,6 +490,7 @@ export default class BasicMessageEditor extends React.Component {
     }
 
     componentWillUnmount() {
+        document.removeEventListener("selectionchange", this._onSelectionChange);
         this._editorRef.removeEventListener("input", this._onInput, true);
         this._editorRef.removeEventListener("compositionstart", this._onCompositionStart, true);
         this._editorRef.removeEventListener("compositionend", this._onCompositionEnd, true);