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/structures/UserSettings.js

@@ -63,6 +63,7 @@ const gHVersionLabel = function(repo, token='') {
 const SIMPLE_SETTINGS = [
     { id: "urlPreviewsEnabled" },
     { id: "autoplayGifsAndVideos" },
+    { id: "alwaysShowEncryptionIcons" },
     { id: "hideReadReceipts" },
     { id: "dontSendTypingNotifications" },
     { id: "alwaysShowTimestamps" },