Hide inline encryption icons except when hovering over a message

Closes https://github.com/vector-im/riot-web/issues/2882

This is a redo of https://github.com/matrix-org/matrix-react-sdk/pull/1707 (see associated discussion there and here: https://github.com/vector-im/riot-web/pull/5988)

I tried several times to resolve the conflicts correctly, but could not. Thus, fresh PR.

src/components/views/rooms/EventTile.js

@@ -742,7 +742,11 @@ function E2ePadlockUnencrypted(props) {
 }
 
 function E2ePadlock(props) {
-    return <img className="mx_EventTile_e2eIcon" {...props} />;
+    if (SettingsStore.getValue("alwaysShowEncryptionIcons")) {
+        return <img className="mx_EventTile_e2eIcon" {...props} />;
+    } else {
+        return <img className="mx_EventTile_e2eIcon" hidden {...props} />;
+    }
 }
 
 module.exports.getHandlerTile = getHandlerTile;