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/settings/Settings.js

@@ -150,6 +150,11 @@ export const SETTINGS = {
         displayName: _td('Autoplay GIFs and videos'),
         default: false,
     },
+    "alwaysShowEncryptionIcons": {
+        supportedLevels: LEVELS_ACCOUNT_SETTINGS,
+        displayName: _td('Always show encryption icons'),
+        default: true,
+    },
     "enableSyntaxHighlightLanguageDetection": {
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,
         displayName: _td('Enable automatic language detection for syntax highlighting'),